wine - ENV.libxml2 needed if no CLT

libxml2 support will be compiled into wine if configure finds it. But
configure will not find it in an 'XCode only' environment.

Adding `ENV.libxml2` to the formula appears to resolve the issue
satisfactorily. (The formula specifies `env :std`.)


'wine without libxml2' breaks at least one Windows InstallShield based
'exe style' installer. InstallShield is widely used to develop Windows
installer packages, so there are probably many Windows installers that
will fail in libxml2's absence.