GLES3/gl3ext.h missing

GLES3/gl3ext.h header is still missing after commit e5a4edcf.

This header in Mesa is quite empty (https://gitlab.freedesktop.org/mesa/mesa/blob/master/include/GLES3/gl3ext.h), but for compatibility libglvnd should provide it. There is at least one consumer which includes this header.

Original bug: https://bugs.gentoo.org/698932