Fix compile w/ clang and IPv6

Submitted by:  Yuri Pankov <yuri.pankov@gmail.com>
               via freebsd ports/164420

Backport of r1235047 from trunk.


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/branches/1.5.x@1528869 13f79535-47bb-0310-9956-ffa450edef68