Mark memcache_pconnect as an external function for static analysis

Summary:
If you don't have the memcache extension installed, this function won't exist.

We have only one callsite from the extension and are unlikely (probably?) to accumulate more, so just external it rather than whitelisting the whole extension in arc.

Test Plan: idk shrugz~

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3989