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/Buildings/PrimaryBuilding.cs

@@ -32,6 +32,7 @@ namespace OpenRA.Mods.Common.Traits
 		[Desc("The condition to grant to self while this is the primary building.")]
 		public readonly string PrimaryCondition = null;
 
+		[NotificationReference("Speech")]
 		[Desc("The speech notification to play when selecting a primary building.")]
 		public readonly string SelectionNotification = null;