add missing GC traversal-function registration

Related to places and place messages.

(Bug found by running tests in GC-stress mode.)

racket/src/racket/src/type.c

@@ -615,6 +615,7 @@ void scheme_register_traversers(void)
   GC_REG_TRAV(scheme_symbol_type, symbol_obj);
 #ifdef MZ_USE_PLACES  
   GC_REG_TRAV(scheme_serialized_symbol_type, bstring_obj);
+  GC_REG_TRAV(scheme_serialized_keyword_type, bstring_obj);
   GC_REG_TRAV(scheme_place_dead_type, small_object);
 #endif
   GC_REG_TRAV(scheme_keyword_type, symbol_obj);