gdb: add interp::on_traceframe_changed method

Same idea as previous patches, but for traceframe_changed.

Change-Id: Ia473f07d70d57b30aca0094d0e0585d7e0d95637

gdb/interps.c

@@ -527,6 +527,14 @@ interps_notify_solib_unloaded (so_list *so)
   interps_notify (&interp::on_solib_unloaded, so);
 }
 
+/* See interps.h.  */
+
+void
+interps_notify_traceframe_changed (int tfnum, int tpnum)
+{
+  interps_notify (&interp::on_traceframe_changed, tfnum, tpnum);
+}
+
 /* This just adds the "interpreter-exec" command.  */
 void _initialize_interpreter ();
 void