Add missing include for c++11

Change-Id: Ibaf61792254122d0e18337c0ebb03d957b79d367
Reviewed-on: http://review.couchbase.org/29891
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>