Avoid a couple compilation warnings

src/controls/channels.c

@@ -280,6 +280,8 @@ static gboolean _get_mute(MixerControlPlugin * channels)
 /* channels_get_type */
 static String const * _channels_get_type(MixerControlPlugin * channels)
 {
+	(void) channels;
+
 	return "channels";
 }
 

src/controls/mute.c

@@ -168,6 +168,8 @@ static int _mute_get(MixerControlPlugin * mute, va_list properties)
 /* mute_get_type */
 static String const * _mute_get_type(MixerControlPlugin * mute)
 {
+	(void) mute;
+
 	return "mute";
 }