APR DSOs have .dll extension on Cygwin too.

PR: 55587
Submitted by: Carlo Bramini <carlo.bramix libero.it>
Reviewed by: trawick


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/trunk@1529554 13f79535-47bb-0310-9956-ffa450edef68