AudioPane: Fix inconsistent initial state of audio stretching labels

[Pokechu22]

This resulted in the labels being solid black even when audio stretching is disabled the first time the settings are opened, but then properly being greyed out after changing a setting (even the audio backend or DSP emulation engine, not just whether audio stretching is enabled).

Old:

New:

See the code in SaveSettings:

dolphin/Source/Core/DolphinQt/Settings/AudioPane.cpp

Lines 314 to 321 in 74abf48

	// Stretch
	Config::SetBaseOrCurrent(Config::MAIN_AUDIO_STRETCH, m_stretching_enable->isChecked());
	Config::SetBaseOrCurrent(Config::MAIN_AUDIO_STRETCH_LATENCY, m_stretching_buffer_slider->value());
	m_stretching_buffer_label->setEnabled(m_stretching_enable->isChecked());
	m_stretching_buffer_slider->setEnabled(m_stretching_enable->isChecked());
	m_stretching_buffer_indicator->setEnabled(m_stretching_enable->isChecked());
	m_stretching_buffer_indicator->setText(
	tr("%1 ms").arg(Config::Get(Config::MAIN_AUDIO_STRETCH_LATENCY)));