DolphinQt/MainWindow: Surround prototype of OnSignal() with relevant ifdef

This is only used on Apple and Unix-like machines, so we can enclose the
prototype with an ifdef like the implementation is. This prevents
false-positives about an unimplemented function prototype.

Source/Core/DolphinQt/MainWindow.h

@@ -162,7 +162,10 @@ private:
   void OnBootGameCubeIPL(DiscIO::Region region);
   void OnImportNANDBackup();
   void OnConnectWiiRemote(int id);
+
+#if defined(__unix__) || defined(__unix) || defined(__APPLE__)
   void OnSignal();
+#endif
 
   void OnUpdateProgressDialog(QString label, int progress, int total);