qt5webkit: fix build issue with mesa3d w/out xcb

qt5webkit: fix build issue with mesa3d w/out xcb

The mesa's EGL/eglplatform.h header includes X11 headers unless the flag
MESA_EGL_NO_X11_HEADERS is defined[1].

A build issue happens when mesa3d is selected as then OpenGL EGL backend
but the XCB library is not selected.

This commit tells qmake to pass the cflag MESA_EGL_NO_X11_HEADERS to
make and prevent from including the missing X headers.

The issue QTBUG-66233 is opened in the Qt tracker[1].

Fixes:

	In file included from /home/gportay/src/buildroot/output-qt5.10/host/x86_64-buildroot-linux-gnu/sysroot/usr/include/EGL/egl.h:39:0,
	                 from platform/graphics/opengl/Extensions3DOpenGLES.cpp:33:
	/home/gportay/src/buildroot/output-qt5.10/host/x86_64-buildroot-linux-gnu/sysroot/usr/include/EGL/eglplatform.h:118:22: fatal error: X11/Xlib.h: No such file or directory
	 #include <X11/Xlib.h>
	                      ^
	compilation terminated.
	Makefile.WebCore.Target:93596: recipe for target '.obj/platform/graphics/opengl/Extensions3DOpenGLES.o' failed

[1]: https://github.com/mesa3d/mesa/blob/79ee1b2ff0b85f4eeb4165d23a7943c28d3a3d93/include/EGL/eglplatform.h#L109-L125
[2]: https://bugreports.qt.io/browse/QTBUG-66233

[Peter: simplify logic]
Cc: Julien CORJON <corjon.j@ecagroup.com>
Signed-off-by: Gaël PORTAY <gael.portay@savoirfairelinux.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>