Define __declspec on mingw as well

Fixes some linker issues

Change-Id: I7cf88b5f8a0647d06fe62646c6f83330577da005
Reviewed-on: http://review.couchbase.org/35873
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>