msvc compilation error for libevent2, missing HAVE_EVENT2_DNS_H

If we assume MSVC builds have to use the latest available libevent version (no reason not to since it has to be built from source anyway), then src/win32/orconfig.h requires an additional line:

#define HAVE_EVENT2_DNS_H

otherwise mixing of old and new version code happens, and

3>libtor.lib(dns.obj) : error LNK2019: unresolved external symbol _evdns_set_default_outgoing_bind_address referenced in function _configure_nameservers
3>libtor.lib(dns.obj) : error LNK2019: unresolved external symbol _evtimer_add referenced in function _dns_launch_correctness_checks
3>libtor.lib(dns.obj) : error LNK2019: unresolved external symbol _evtimer_new referenced in function _dns_launch_correctness_checks

Trac:
Username: ultramage