Update imemo_mask to match ruby's

The imemo mask has been `0xf` since Ruby 2.5 /
ccfe37884ab566336380d0f21e15321d6382da8f

This hasn't caused problems yet because the only
possible conflict is `imemo_type = 15`, but it is
not yet used.

Author: byroot

ext/debug/iseq_collector.c

@@ -10,7 +10,7 @@ size_t rb_obj_memsize_of(VALUE);
 // implementation specific.
 enum imemo_type {
     imemo_iseq = 7,
-    imemo_mask = 0x07
+    imemo_mask = 0xf
 };
 
 static inline enum imemo_type