subprojects: Update gvc to latest commit

A crash possibility has been fixed in libgnome-volume-control!25 (merged).

A similar crash was reported on the Settings side, so update to the latest gvc commit to hopefully prevent further crashes.

Related: gnome-shell#7173 (closed)
Related: #2924 (closed)

I closed #2924 (closed) without realizing gvc is not packaged, so we need to update that ourselves. Similar thing happened in gnome-shell!3233 (merged) :)