[WindowsDriver] Fix the EventType type

Terminal.Gui/Drivers/WindowsDriver.cs

@@ -221,7 +221,7 @@ namespace Terminal.Gui {
 			public uint bSetFocus;
 		}
 
-		public enum EventType {
+		public enum EventType : ushort {
 			Focus = 0x10,
 			Key = 0x1,
 			Menu = 0x8,