Updated main.cpp

Fixed render window focus issue.
Game_list had full focus until now. This caused issues when, After starting a game, if we pressed down+enter, the game list would then load the second game from list. By using setfocus() , render window focus was fixed. 
Thanks to @jroweboy for helping.

src/citra_qt/main.cpp

@@ -390,6 +390,7 @@ void GMainWindow::BootGame(const std::string& filename) {
         game_list->hide();
     }
     render_window->show();
+    render_window->setFocus();
 
     emulation_running = true;
     OnStartGame();