PDF quick plugin: fix warning about unused engine arg on static builds

The build for iOS is static.

Change-Id: I3e6c13c920861a2a6bf0079e53e76322bb93935f
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/pdf/quick/plugin.cpp

@@ -71,7 +71,9 @@ public:
 
     void initializeEngine(QQmlEngine *engine, const char *uri) override {
         Q_UNUSED(uri);
-#ifndef QT_STATIC
+#ifdef QT_STATIC
+        Q_UNUSED(engine);
+#else
         engine->addImportPath(QStringLiteral("qrc:/"));
 #endif
     }