citra_qt: Migrate to Qt 5 signal/slot connection syntax where applicable

This is more type-safe than the string-based signal/slot syntax that was
being used. It also makes the connections throughout the UI code consistent.

src/citra_qt/configuration/configure_audio.cpp

@@ -21,8 +21,9 @@ ConfigureAudio::ConfigureAudio(QWidget* parent)
     }
 
     this->setConfiguration();
-    connect(ui->output_sink_combo_box, SIGNAL(currentIndexChanged(int)), this,
-            SLOT(updateAudioDevices(int)));
+    connect(ui->output_sink_combo_box,
+            static_cast<void (QComboBox::*)(int)>(&QComboBox::currentIndexChanged), this,
+            &ConfigureAudio::updateAudioDevices);
 }
 
 ConfigureAudio::~ConfigureAudio() {}