Fix using Cogl from gobject-introspection
Commit 6885c377 changed how Cogl symbols are imported, which accidentally hid two symbols that are required for Cogl to be usable from introspection.
I'm undecided which is uglier:
- export symbols from a private header
- make symbols that should be private public
I went with the least intrusive change, that is leaving the declarations where they are.
Fixes #1146 (closed)