www/webkit2-gtk3: unbreak with libglvnd >= 1.3.4

Source/WebCore/platform/graphics/x11/PlatformDisplayX11.cpp:132:5: error: unknown type name 'XVisualInfo'

XVisualInfo visualTemplate;
^

Source/WebCore/platform/graphics/x11/PlatformDisplayX11.cpp:136:5: error: unknown type name 'XVisualInfo'

XVisualInfo* visualInfo = XGetVisualInfo(m_display, VisualScreenMask, &visualTemplate, &visualCount);
^

Source/WebCore/platform/graphics/x11/PlatformDisplayX11.cpp:136:57: error: use of undeclared identifier 'VisualScreenMask'

XVisualInfo* visualInfo = XGetVisualInfo(m_display, VisualScreenMask, &visualTemplate, &visualCount);
                                                    ^

PR: 258269