st/button: Add the clicked button to virtual function signature

clicked signal includes a clicked mouse button parameter, but the vfunc
signature doesn't include it, so it won't be passed to the functions when
the signal is emitted.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/559

src/st/st-button.h

@@ -40,7 +40,7 @@ struct _StButtonClass
   void (* transition) (StButton     *button);
 
   /* signals */
-  void (* clicked) (StButton *button);
+  void (* clicked) (StButton *button, int clicked_button);
 };
 
 StWidget    *st_button_new             (void);