egl: Add header to import symbols from new EGL extensions

Waffle uses some EGL extensions that are fairly recent, so recent that some
systems' EGL headers lack the extensions. This header imports the needed
symbols from those extensions.

The only extension imported by this commit is EGL_KHR_create_context.

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>
khronos: Remove imported headers from Khronos

This removes the imported headers:
    EGL/egl.h
    EGL/eglext.h
    EGL/eglplatform.h
    KHR/khrplatform.h

I was unwise to use imported headers for a system library. If EGL were
to change its ABI, and Waffle were linked against a libEGL that exports
the new ABI, then Waffle would risk undefined behavior due to using EGL
headers from the old ABI.

Hopefully, Khronos would never introduce such a situation. But let's not
rely on hope.

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>