tuba: support cross compilation

gobject-introspection is needed in nativeBuildInputs to fix
"error: Package <...> not found in specified Vala API directories
or GObject-Introspection GIR directories".

pkgs/applications/misc/tuba/default.nix

···
 , json-glib
 , glib
 , glib-networking
+, gobject-introspection
 , gtksourceview5
 , libxml2
 , libgee
···
     python3
     wrapGAppsHook4
     desktop-file-utils
+    gobject-introspection
   ];
 
   buildInputs = [