webkitgtk: Fix build when x11 is not in distro features
ANGLE's copy of khrplatform.h is not uptodate with mesa/khronos khrplatform.h
which uses MESA_EGL_NO_X11_HEADERS which is now renamed in newer headers to
EGL_NO_X11 from mesa/khronos headers, however this define is
relatively new and 3D stacks do not have this adopted but apps
like qemu and bunch of others depend on it, I guess they assume mesa.
One can argue that its better to fix the 3D stacks to behave
like mesa but this means every BSP using these stacks will need to
carry such a fix.
