Since 248a76f5eac7e5ae4fb45208577f9a55360812a7 cogl.h can no longer be
included in internal source files so the WGL winsys was no longer
compiling.
Reviewed-by: Robert Bragg <robert@linux.intel.com>
(cherry picked from commit 91af97a2a27ab5ad3e7eaabebd03503b685d4d42)