utils: add ESource* argument to ask-recurrence-modification-type

Earlier the argument was ESource. Changed
the argument to ESource* for the function
to correctly fetch "client" data from the
source.

https://bugzilla.gnome.org/show_bug.cgi?id=782755

src/gcal-utils.h

@@ -141,6 +141,6 @@ gboolean        is_source_enabled                               (ESource
 
 gboolean        ask_recurrence_modification_type                (GtkWidget             *parent,
                                                                  ECalObjModType        *modtype,
-                                                                 ESource                source);
+                                                                 ESource               *source);
 
 #endif // __GCAL_UTILS_H__