Use "@" to silence "GC list" warnings from "apc_store()" and "apcu_store()"

Summary:
Fixes T13525. Since D21044, the intermittent GC list warnings are treated more severely and can become user-visible errors.

Silence them, since this seems to be the only realistic response in most versions of APC/APCu.

Test Plan: Will deploy.

Maniphest Tasks: T13525

Differential Revision: https://secure.phabricator.com/D21179