Avoid warnings for discarded const under some platforms like OSF1 and FreeBSD

2.2.x where dlopen is defined to have a "char *" as its first argument instead
of a "const char *".


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@80961 13f79535-47bb-0310-9956-ffa450edef68