Explicitly include mod_ldap before mod_authnz_ldap

Fixes this error
[Tue Sep 09 10:40:20.589086 2014] [authnz_ldap:error] [pid 27819] AH01749: Module mod_ldap missing. Mod_ldap (aka. util_ldap) must be loaded in order for mod_authnz_ldap to function properly