Fix gdb support

Author: markshannon

Tools/gdb/libpython.py

@@ -1755,8 +1755,8 @@ def is_waiting_for_gil(self):
     def is_gc_collect(self):
         '''Is this frame a collector within the garbage-collector?'''
         return self._gdbframe.name() in (
-            'collect', 'gc_collect_full',
-            'gc_collect_young', 'gc_collect_increment'
+            'collect', 'gc_collect_full', 'gc_collect_main',
+            'gc_collect_young', 'gc_collect_increment',
         )
 
     def get_pyop(self):