[class-parse] Ignore Java Native libraries.

[jpobst]

Fixes: #860

.jnilib files seem to be some sort of Java Native file for Mac . It starts with the magic token of 0xcafebabe but is not the type of compiled .class file we understand and are interested in.

Trying to parse it results in:

System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.

  StackTrace:
   at System.ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument argument, ExceptionResource resource)
   at System.Collections.Generic.List`1.get_Item(Int32 index)
   at Xamarin.Android.Tools.Bytecode.AttributeInfo.CreateFromStream(ConstantPool constantPool, Stream stream) in 
 Xamarin.Android.Tools.Bytecode\AttributeInfo.cs:line 101

We should ignore this file in a .jar.