Make qtbase compile for WASM with Werror

There are unused variable warnings which cause the compilation to
fail when Werror is used in qpermissions_wasm.cpp

Change-Id: I2cf2035f52f0a5d570a56dcd4c82bf2ecf3006c7
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/corelib/kernel/qpermissions_wasm.cpp

@@ -131,7 +131,7 @@ namespace
             return cb(Qt::PermissionStatus::Denied);
 
         qstdweb::PromiseCallbacks queryCallbacks;
-        queryCallbacks.thenFunc = [device, cb](val mediaStream)
+        queryCallbacks.thenFunc = [device, cb](val)
         {
             updatePermission(device, wapiGranted, cb);
         };