meson: disable warnings for unused static inline functions with clang
These prevent us from using G_DECLARE_*_TYPE macros outside of header files, as these generate static inline functions for boilerplate macros (eg GST_IS_).
https://gitlab.gnome.org/GNOME/glib/merge_requests/1062 has been proposed in GLib, but we can use this as a workaround for now.
Taken from https://lore.kernel.org/patchwork/patch/796288/
If this is OK, I'll push similar commits in all our repos.