Avoid unused param with clang

src/rhi/qrhimetal.mm

@@ -274,6 +274,8 @@ struct QMetalSwapChainData
 
 QRhiMetal::QRhiMetal(QRhiMetalInitParams *params, QRhiMetalNativeHandles *importDevice)
 {
+    Q_UNUSED(params);
+
     d = new QRhiMetalData(this);
 
     importedDevice = importDevice != nullptr;