meson-build: Compile gsettings schemas

This is actually a quite important step in self-compile/install and as mentioned NixOS packaging actually relies on this. 😉

In autotools build this is done in @GSETTINGS_RULES@.

Not doing this can lead to completely broken app (spamming e.g. mousepad_setting_get: code should not be reached and only showing a blank window).

(So far I think mousepad is the only project that need this)