httpdunit: fix check.h on Windows

Check seems to have trouble with type definitions when using a
CMake+Windows toolchain. Include apr.h before check.h to ensure pid_t
exists.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/httpdunit@1796205 13f79535-47bb-0310-9956-ffa450edef68