vm_dump.c: fix warning about constness

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63969 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

vm_dump.c

@@ -141,7 +141,8 @@ void
 rb_vmdebug_stack_dump_raw(const rb_execution_context_t *ec, const rb_control_frame_t *cfp)
 {
 #if 0
-    VALUE *sp = cfp->sp, *ep = cfp->ep;
+    VALUE *sp = cfp->sp;
+    const VALUE *ep = cfp->ep;
     VALUE *p, *st, *t;
 
     fprintf(stderr, "-- stack frame ------------\n");