x11-toolkits/qt5-gui: unbreak with libglvnd >= 1.3.4

executing config test egl-x11
[...]

main.cpp:13:14: error: cannot initialize a variable of type 'Display *' (aka '_XDisplay *') with an rvalue of type 'EGLNativeDisplayType' (aka 'void *')

Display *dpy = EGL_DEFAULT_DISPLAY;
         ^     ~~~~~~~~~~~~~~~~~~~

[...]
QPA backends:

XCB:
  GL integrations:
    EGL-X11 Plugin ..................... no

[...]

> Checking for pkg-plist issues (check-plist)

> Parsing plist

> Checking for items in STAGEDIR missing from pkg-plist

> Checking for items in pkg-plist which are not in STAGEDIR

Error: Missing: %%QT_CMAKEDIR%%/Qt5Gui/Qt5Gui_QXcbEglIntegrationPlugin.cmake
Error: Missing: %%QT_PLUGINDIR%%/xcbglintegrations/libqxcb-egl-integration.so

> Error: Plist issues found.

• Error code 1

PR: 258269
See also: https://bugs.gentoo.org/810343