Delete NS_ADDRESS definition from mod_multicast.erl

While trying to compile mod_multicast I had this error:

~/ejabberd-contrib/mod_multicast$ ./build.sh
Recompile: src/mod_multicast
src/mod_multicast.erl:74: redefining macro 'NS_ADDRESS'

This commit fixes the problem. I think this change should be applied to 2.1.x branch as well.