citra-qt: Pass string by const reference

src/citra_qt/main.cpp

@@ -207,7 +207,7 @@ void GMainWindow::OnDisplayTitleBars(bool show)
     }
 }
 
-void GMainWindow::BootGame(std::string filename) {
+void GMainWindow::BootGame(const std::string& filename) {
     LOG_INFO(Frontend, "Citra starting...\n");
 
     // Initialize the core emulation

src/citra_qt/main.h

@@ -55,7 +55,7 @@ signals:
     void EmulationStopping();
 
 private:
-    void BootGame(std::string filename);
+    void BootGame(const std::string& filename);
     void ShutdownGame();
 
     void closeEvent(QCloseEvent* event) override;