x11/nwg-launchers: unbreak build with Clang 13

In file included from ../bar/bar.cc:12:
../common/nwg_classes.h:200:33: error: no viable constructor or deduction guide for deduction of template arguments of 'array'

static constexpr std::array MAGIC { 'i', '3', '-', 'i', 'p', 'c' };
                            ^

/usr/include/c++/v1/__tuple:219:64: note: candidate function template not viable: requires 1 argument, but 6 were provided
template <class _Tp, size_t _Size> struct _LIBCPP_TEMPLATE_VIS array;

^

/usr/include/c++/v1/__tuple:219:64: note: candidate function template not viable: requires 0 arguments, but 6 were provided
In file included from ../bar/bar.cc:12:
../common/nwg_classes.h:203:52: error: invalid operands to binary expression ('const auto' and 'unsigned long')

static constexpr auto HEADER_SIZE = MAGIC_SIZE + 2 * sizeof(std::uint32_t);
                                    ~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~

../common/nwg_classes.h:325:25: error: implicit instantiation of undefined template 'std::array<bool, 4>'

std::array<bool, 4> edges{ 0, 0, 0, 0 };
                    ^

/usr/include/c++/v1/__tuple:219:64: note: template is declared here
template <class _Tp, size_t _Size> struct _LIBCPP_TEMPLATE_VIS array;

^

In file included from ../bar/bar.cc:12:
../common/nwg_classes.h:326:25: error: implicit instantiation of undefined template 'std::array<int, 4>'

std::array<int,  4> margins{ 0, 0, 0, 0 };
                    ^

/usr/include/c++/v1/__tuple:219:64: note: template is declared here
template <class _Tp, size_t _Size> struct _LIBCPP_TEMPLATE_VIS array;

^

In file included from ../bar/bar.cc:12:
../common/nwg_classes.h:342:16: error: no viable constructor or deduction guide for deduction of template arguments of 'array'

std::array edges_ {
           ^

/usr/include/c++/v1/__tuple:219:64: note: candidate function template not viable: requires 1 argument, but 4 were provided
template <class _Tp, size_t _Size> struct _LIBCPP_TEMPLATE_VIS array;

^

/usr/include/c++/v1/__tuple:219:64: note: candidate function template not viable: requires 0 arguments, but 4 were provided
../bar/bar.cc:106:9: error: cannot initialize object parameter of type 'CommonWindow' with an expression of type 'BarWindow'

window.set_background_color(background_color);
^~~~~~

../bar/bar.cc:133:9: error: cannot initialize object parameter of type 'Gtk::Container' with an expression of type 'BarWindow'

window.show_all_children();
^~~~~~

../bar/bar.cc:134:16: error: no matching member function for call to 'show'

window.show(hint::Fullscreen);
~~~~~~~^~~~

../common/nwg_classes.h:364:22: note: candidate function template not viable: no known conversion from 'BarWindow' to 'PlatformWindow' for object argument
void PlatformWindow::show(Hint h) {

^

../bar/bar.cc:141:21: error: no matching member function for call to 'run'

return app->run(window);
       ~~~~~^~~

/usr/local/include/gtkmm-3.0/gtkmm/application.h:480:7: note: candidate function not viable: no known conversion from 'BarWindow' to 'Gtk::Window &' for 1st argument

int run(Window& window);
    ^

/usr/local/include/gtkmm-3.0/gtkmm/application.h:496:7: note: candidate function not viable: requires 0 arguments, but 1 was provided

int run();
    ^

/usr/local/include/gtkmm-3.0/gtkmm/application.h:438:7: note: candidate function not viable: requires 2 arguments, but 1 was provided

int run(int argc, char** argv);
    ^

/usr/local/include/gtkmm-3.0/gtkmm/application.h:459:7: note: candidate function not viable: requires 3 arguments, but 1 was provided

int run(Window& window, int argc, char** argv);
    ^

Reported by: antoine (via bug 258209 exp-run)