declare rb_gc_abort_threads.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@9766 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

gc.c

@@ -1002,6 +1002,8 @@ free_unused_heaps(void)
     }
 }
 
+void rb_gc_abort_threads(void);
+
 static void
 gc_sweep(void)
 {