gcal-window: Declare 'view' as type guint32

Earlier 'view' was declared as a 'guint' but was used as 'gint'.
This patch changes the declaration of 'view' to gint32.

https://bugzilla.gnome.org/show_bug.cgi?id=778299

src/gcal-window.c

@@ -327,7 +327,7 @@ on_view_action_activated (GSimpleAction *action,
                           gpointer       user_data)
 {
   GcalWindow *window = GCAL_WINDOW (user_data);
-  guint view;
+  gint32 view;
 
   view = g_variant_get_int32 (param);