ringtone: prevent attempt to play ringtone file on UWP

- libsndfile was removed from the UWP and incoming ringtones
  are handled within the client. This patch conditionally prevents
  playRingtone from being called, so that an empty buffer is not
  played.

Change-Id: Ib6f6fcfe4cb53eaf5114c344b51444c01940df2a
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>

src/manager.cpp

@@ -1890,7 +1890,9 @@ Manager::incomingCall(Call &call, const std::string& accountId)
 
     if (not hasCurrentCall()) {
         call.setState(Call::ConnectionState::RINGING);
+#ifndef RING_UWP
         playRingtone(accountId);
+#endif
     }
 
     pimpl_->addWaitingCall(callID);