Ignore extra dll files

Change-Id: If80068f0a93ea14eba3d00212f9b10e9677d5dc2
Reviewed-on: http://review.couchbase.org/30085
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>