Include strings.h to fix build break on win32

Change-Id: Ia1a031f503db188b54141a5a6f27c752fe6a3dda
Reviewed-on: http://review.couchbase.org/31192
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>