Fix constness problem reported by gcc:

mod_cgi.c: In function `include_cmd':
mod_cgi.c:812: warning: passing arg 1 of pointer to function from incompatible
pointer type

Reported by:	Ian's autobuild


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@91062 13f79535-47bb-0310-9956-ffa450edef68