egl: avoid local modifications for eglext.h Khronos standard header file

egl: avoid local modifications for eglext.h Khronos standard header file

Move differences in eglextchromium.h header file, then provide the same header than libglvnd-1.2
So program that omit to include eglextchromium.h will fail to build with both mesa and libglvnd headers.

Fixes: a0a8109f "include: add the definition of EGL_EXT_image_flush_external"
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>