build: Include GioUnix-2.0 when scanning the headers

Cinnamon exposes in the headers types that are part of GioUnix
(DesktopAppInfo), so we should make it an explicit dependency of the
introspection parser, or we may not be able to parse these types.

Author: 3v1n0

src/meson.build

@@ -199,6 +199,7 @@ executable(
 )
 
 cinnamon_gir_includes = [
+    'GioUnix-2.0',
     'Clutter-0',
     'ClutterX11-0',
     'CoglPango-0',