girepository: Make _blob_is_registered_type static inline
This follows the usage in the glib codebase and recommendations at main/docs/toolchain-requirements.md
It also fixes a build warning with ancient gcc 4.8: ../girepository/gitypelib-internal.h:202:1: warning: no previous prototype for ‘_blob_is_registered_type’ [-Wmissing-prototypes]
Edited by Olivier Blin