tests: Fix a double-unref in the GSettings unit tests

g_settings_schema_source_get_default() is (transfer none), not (transfer
full).

Spotted by Marvin Schmidt.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=779265

gio/tests/gsettings.c

@@ -2373,10 +2373,8 @@ test_schema_list_keys (void)
                             "farewell",
                             NULL));
 
-
   g_strfreev (keys);
   g_settings_schema_unref (schema);
-  g_settings_schema_source_unref (src);
 }
 
 static void