QNativeSocketEnginePrivate::nativeAccept(): Add missing break

Fixes GCC 7 warning:
socket\qnativesocketengine_win.cpp:888:21: warning: this statement may fall through [-Wimplicit-fallthrough=]

Change-Id: Idb4dce511b3ab634339ac1283d88def283a92028
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/network/socket/qnativesocketengine_win.cpp

@@ -886,6 +886,7 @@ int QNativeSocketEnginePrivate::nativeAccept()
             break;
         case WSAENETDOWN:
             setError(QAbstractSocket::NetworkError, NetworkUnreachableErrorString);
+            break;
         case WSAENOTSOCK:
             setError(QAbstractSocket::SocketResourceError, NotSocketErrorString);
             break;