Add a lint test for audio notifications.

Only traits are linted - the UI still hardcodes
too many audio references for this to be worthwhile.

OpenRA.Mods.Common/Traits/ProductionParadrop.cs

@@ -27,6 +27,7 @@ namespace OpenRA.Mods.Common.Traits
 		[Desc("Sound to play when dropping the unit.")]
 		public readonly string ChuteSound = null;
 
+		[NotificationReference("Speech")]
 		[Desc("Notification to play when dropping the unit.")]
 		public readonly string ReadyAudio = null;