Some ClutterScrollEvent methods not properly exposed through GIR

Accessing clutter_event_get_scroll_source and clutter_event_get_scroll_finish_flags from GJS gives the following error:

Could not locate clutter_event_get_scroll_finish_flags: 'clutter_event_get_scroll_finish_flags': /nix/store/0fx5pgrvyl2fs5is1c123d0ij6xiwscb-mutter-3.28.2/lib/mutter/libmutter-clutter-2.so: undefined symbol: clutter_event_get_scroll_finish_flags

Comparison with how the other functions are declared reveals that these are missing a CLUTTER_AVAILABLE_IN_1_* macro.

Adding the macro fixes the problem - though both methods always return 0 on my system.

PS: I use nixos, but https://github.com/linuxmint/Cinnamon/issues/6376 indicates that it's not a nixos-specific problem.