Mark mouseEvent in wasm compositor as unused

If this is not there, compiling fails for all-warnings-are-errors,
which seems to be enabled by default.

Change-Id: Id4cafaa635be201b649012ce8fc75f7cb5fb88a3
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

src/plugins/platforms/wasm/qwasmcompositor.cpp

@@ -1341,6 +1341,7 @@ void QWasmCompositor::enterWindow(QWindow *window, const QPoint &localPoint, con
 }
 bool QWasmCompositor::processMouseEnter(const EmscriptenMouseEvent *mouseEvent)
 {
+    Q_UNUSED(mouseEvent)
     // mouse has entered the canvas area
     mouseInCanvas = true;
     return true;