vm_insnhelper.c: suppress a warning

* vm_insnhelper.c (vm_yield_with_cfunc): cast to suppress a
  warning by VC.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52144 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

vm_insnhelper.c

@@ -2320,7 +2320,7 @@ vm_yield_with_cfunc(rb_thread_t *th, const rb_block_t *block, VALUE self,
 	data = SYM2ID((VALUE)ifunc);
     }
     else {
-	func = ifunc->func;
+	func = (rb_block_call_func *)ifunc->func;
 	data = (VALUE)ifunc->data;
     }
     val = (*func)(arg, data, argc, argv, blockarg);