Update C# conformance failure list.

conformance/failure_list_csharp.txt

@@ -0,0 +1,4 @@
+Required.ProtobufInput.IllegalZeroFieldNum_Case_0
+Required.ProtobufInput.IllegalZeroFieldNum_Case_1
+Required.ProtobufInput.IllegalZeroFieldNum_Case_2
+Required.ProtobufInput.IllegalZeroFieldNum_Case_3