Fix mingw build. It does support only native plugin via libcouchbase

Change-Id: Ief0ec2480e3e6c1507ff1b8fd70b2812a8cfccd6
Reviewed-on: http://review.couchbase.org/24503
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>