Remove a warning from msg_receive function

Summary: Remove a warning from `msg_receive` function for consistency with php:
https://github.com/php/php-src/blob/a770d29df74515197c76efdf1a64d9794c27b4af/ext/sysvmsg/sysvmsg.c#L377-L379
Closes https://github.com/facebook/hhvm/pull/4325

Reviewed By: @fredemmott

Differential Revision: D1700537

Signature: t1:1700537:1417318587:0560094192f76da223b5c379bd9da929f28f1b1e