Add HAVE_EVENT2_DNS_H to MSVC orconfig.h

Fix for bug 7313

src/win32/orconfig.h

@@ -248,3 +248,4 @@
 #define FLEXIBLE_ARRAY_MEMBER 0
 #define HAVE_EVENT2_EVENT_H
 #define SHARE_DATADIR ""
+#define HAVE_EVENT2_DNS_H