wasm: fix unused variable warning/error

Change-Id: If6dabc94f6d64695a0b28e73061274f8ea1c4ed6
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/network/access/qhttpnetworkconnection.cpp

@@ -258,9 +258,9 @@ void QHttpNetworkConnectionPrivate::prepareRequest(HttpMessagePair &messagePair)
 
     // add missing fields for the request
     QByteArray value;
+#ifndef Q_OS_WASM
     // check if Content-Length is provided
     QNonContiguousByteDevice* uploadByteDevice = request.uploadByteDevice();
-#ifndef Q_OS_WASM
     if (uploadByteDevice) {
         const qint64 contentLength = request.contentLength();
         const qint64 uploadDeviceSize = uploadByteDevice->size();