Remove GL_GLEXT_PROTOTYPES guards from non-ext headers.

Remove GL_GLEXT_PROTOTYPES guards from non-ext headers.

A earlier sync with the Khronos headers added _extension_ prototype
guards to all the GLES2/3/31/32 core entry points. Effectively breaking
all the applications that aim to be portable and do not set the define.

The issue has been reported to Khronos (internal bugzilla #14206) and is
being worked on. Until updated/fixed headers are released locally fix
the issue.

The following report is when building weston.
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97773

Cc: Armin Krezović <krezovic.armin@gmail.com>
Cc: Emmanuel Gil Peyrot <emmanuel.peyrot@collabora.com>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Fixes: 6a5504de2fb ("Update Khronos-supplied headers to r33100")
Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>