[AudioToolBox] Add nullability to files that were forgotten. (#17185)

Author: mandel-macaque

src/AudioToolbox/AudioSessions.cs

@@ -32,6 +32,8 @@
 using CoreFoundation;
 using ObjCRuntime;
 
+#nullable enable
+
 namespace AudioToolbox {
 
 	public enum AudioSessionErrors { // Implictly cast to OSType 

src/AudioToolbox/Enums.cs

@@ -2,6 +2,8 @@
 using Foundation;
 using ObjCRuntime;
 
+#nullable enable
+
 namespace AudioToolbox {
 	[NoWatch]
 	[TV (10, 0)]