WinSvc: define virtual d'tor

The Windows Service class did not specify its destructor as virtual, but
has other virtual function. Not specifying the d'tor virtual is an
anti-pattern in this case.

Signed-off-by: Heiko Hund <heiko@openvpn.net>

openvpn/win/winsvc.hpp

@@ -59,6 +59,8 @@ class Service
         checkpoint = 1;
     }
 
+    virtual ~Service() = default;
+
     bool is_service() const
     {
         return bool(service);