Build fixes for GTK2.

Apparently I haven't tried a GTK2 build since the most recent set of
GTK-related code reorganisation. Some functions that were ifdef'ed out
in GTK3 builds were now unused even in GTK2 builds (and, because they
were also declared static, caused a -Werror build failure); and the
pointless stub version of gtkapp.c was missing a stub version of a
recently added function referred to from another module.

unix/gtkapp.c

@@ -104,6 +104,7 @@ void launch_duplicate_session(Conf *conf) {}
 void launch_new_session(void) {}
 void launch_saved_session(const char *str) {}
 void session_window_closed(void) {}
+void window_setup_error(const char *errmsg) {}
 #else /* GTK_CHECK_VERSION(3,0,0) */
 
 extern const int use_event_log;