build: fix building introspection data

This fixes the build with --enable-introspection. I'm not sure why
g-ir-scanner seems to parse all public headers in isolation instead of
being able take a more limited list of top-level public headers and
automatically parse all necessary #include directives but this means we
have to special case how we define and undefine __COGL_H_INSIDE__ to
subvert the guards we have in place for detecting misuse of the headers.

Reviewed-by: Neil Roberts <neil@linux.intel.com>