include: use eglext.h Khronos standard header file
Move differences in eglmesaext.h header file, then provide the same header than libglvnd-1.2 So program that omit to include eglmesaext.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