OS/2: Provide a definition of AP_PLATFORM_REWRITE_ARGS_HOOK.

NULL for now as ap_mpm_rewrite_args()'s -k function doesn't currently work on
OS/2.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@956859 13f79535-47bb-0310-9956-ffa450edef68