Remove `G_GNUC_CONST` from *get_type declarations

[correctmost]

GLib recently removed G_GNUC_CONST attributes from its *get_type declarations to avoid miscompilations with GCC trunk:

• https://gitlab.gnome.org/GNOME/glib/-/merge_requests/5223
• https://gitlab.gnome.org/GNOME/glib/-/work_items/3984

I believe similar changes are needed in this repo and many others in the linuxmint org.

More background reading:

• https://blogs.gnome.org/mcatanzaro/2026/06/29/your-_get_type-function-is-not-g_gnuc_const-part-two/
• https://gcc.gnu.org/onlinedocs/gcc/Common-Attributes.html#index-const