Fix Darwin OS (but not macOS) build - mark unusued parameter

... with Q_UNUSED.

Change-Id: I3736c179381deb08cec9c2c399aaf8a24d8d7f0b
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/platformsupport/fontdatabases/mac/qcoretextfontdatabase.mm

@@ -521,6 +521,8 @@ static void addExtraFallbacks(QStringList *fallbackList)
     // add Apple Symbols to cover those too.
     if (!fallbackList->contains(QStringLiteral("Apple Symbols")))
         fallbackList->append(QStringLiteral("Apple Symbols"));
+#else
+    Q_UNUSED(fallbackList)
 #endif
 }