Fix missing limits.h header, required for 2.4.next release,

masked on many platforms by intrinsic limits.h inclusion by
other headers.

Backports: r1736243

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1736253 13f79535-47bb-0310-9956-ffa450edef68