According to gmacros.h, G_STATIC_ASSERT is undefined for __GI_SCANNER__
because it causes confusion. Which is understandable, and so is the
complaint about the lingering G_STATIC_ASSERT after preprocessing.

This problem seems to be unique to just clutter-deform-effect.h because
it's the only header file containing a top-level G_STATIC_ASSERT. The
rest are all in C files.

Fixes: 138e5d4f
Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/3591
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3898>