tweak the const-ness of MODSSL_INFO_CB_ARG_TYPE based on the OpenSSL

version

this resolves some warnings with RH 8 (OpenSSL 0.9.6)
and some errors with AIX's native compiler (OpenSSL 0.9.6)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@99678 13f79535-47bb-0310-9956-ffa450edef68