Define EGL_NO_X11 everywhere were we also define MESA_EGL_NO_X11_HEADERS, EGL_NO_X11 is the MESA_EGL_NO_X11_HEADERS equivalent for the egl headers shipped with libglvnd.

This fixes the xserver not building with the libglvnd-1.2.0 headers:

In file included from /usr/include/EGL/eglplatform.h:128,
                 from /usr/include/epoxy/egl_generated.h:11,
                 from /usr/include/epoxy/egl.h:46,
                 from glamor_priv.h:43,
                 from glamor_composite_glyphs.c:25:
/usr/include/X11/Xlib.h:222:2: error: conflicting types for 'GC'
  222 | *GC;
      |  ^~
In file included from glamor.h:34,
                 from glamor_priv.h:32,
                 from glamor_composite_glyphs.c:25:
../include/gcstruct.h:282:3: note: previous declaration of 'GC' was here
  282 | } GC;
      |   ^~