Fix not emitting PropertyChanged for SpeakerGain/MicrophoneGain

Currently only the deprecated SpeakerGainChanged/MicrophoneGainChanged are
being emitted when some application set then locally.
diff --git a/audio/headset.c b/audio/headset.c
index 243a962..f55e7e7 100644
--- a/audio/headset.c
+++ b/audio/headset.c
@@ -1892,7 +1892,7 @@
 	struct audio_device *device = data;
 	struct headset *hs = device->headset;
 	struct headset_slc *slc = hs->slc;
-	const char *signal;
+	const char *signal, *property;
 	DBusMessage *reply;
 	int err;
 
@@ -1918,6 +1918,7 @@
 
 		slc->sp_gain = gain;
 		signal = "SpeakerGainChanged";
+		property = "SpeakerGain";
 	} else {
 		if (slc->mic_gain == gain) {
 			DBG("Ignoring no-change in microphone gain");
@@ -1926,6 +1927,7 @@
 
 		slc->mic_gain = gain;
 		signal = "MicrophoneGainChanged";
+		property = "MicrophoneGain";
 	}
 
 	if (hs->state != HEADSET_STATE_PLAYING)
@@ -1944,6 +1946,10 @@
 			DBUS_TYPE_UINT16, &gain,
 			DBUS_TYPE_INVALID);
 
+	emit_property_changed(conn, device->path,
+			AUDIO_HEADSET_INTERFACE, property,
+			DBUS_TYPE_UINT16, &gain);
+
 	return reply;
 }