This makes sure that EGL_EGLEXT_PROTOTYPES is defined in the
same places as GL_GLEXT_PROTOTYPES.

BUG=angleproject:1190

Change-Id: I0e55aad5728df765bff495822570e7590b157d68
Reviewed-on: https://chromium-review.googlesource.com/315208Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Austin Kinross <aukinros@microsoft.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>