MB-100: prevent repeated inclusions of couch_db header

Change-Id: I839c21524463a93f4aa56798ca67acf0950e64e7
Reviewed-on: http://review.couchbase.org/25406
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>