ancdata.c: suppress warnings

* ext/socket/ancdata.c (bsock_sendmsg_internal): suppress warnings on
  platforms where msghdr does not have msg_control.

* ext/socket/ancdata.c (bsock_recvmsg_internal): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40536 b2dd03c8-39d4-4d8f-98ff-823fe69b080e