Don't declare 'memcached_compat_st' as LIBCOUCHBASE_API

This makes visual studio sad and whiny. It also doesn't make sense

Change-Id: I4ba43d83f5cb51cee693bfc4e4b28f7c70981a26
Reviewed-on: http://review.couchbase.org/37342
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>