This makes it so that cogl-sdl.h is a top-level header no longer
automatically included by cogl.h. This avoids lots of warnings building
the conformance tests and examples due to SDL.h warning when
__STRICT_ANSI__ isn't defined.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit f7536985437dc85c26b33d1bbe1b7f3d4b32476a)