gobject_gdb.py: fix regression caused by bfbe7127d5

commit bfbe7127d5 which did a code refactor in
gobject_gdb.py introduced a bug by failing to
return the signal name when a signal had no
'detail', this was preventing pretty printing
name for signals with no 'detail'.

gobject/gobject_gdb.py

@@ -215,7 +215,7 @@ class SignalFrame(FrameDecorator):
         if detail is not None:
             return signal + ":" + detail
         else:
-            return detail
+            return signal
 
     def function(self):
         instances = []