[unity] Fixed a warning occurring in newer Unity versions, two static extension method classes named `SkeletonExtensions` in different namespaces but the same file.

spine-unity/Assets/Spine/Runtime/spine-unity/Utility/SkeletonExtensions.cs

@@ -442,7 +442,7 @@ namespace Spine {
 		}
 	}
 
-	public static class SkeletonExtensions {
+	public static class SpineSkeletonExtensions {
 		public static bool IsWeighted (this VertexAttachment va) {
 			return va.bones != null && va.bones.Length > 0;
 		}