Add GeneratedMessageTest.Proto2.cs to EXTRA_DIST

Makefile.am

@@ -107,6 +107,7 @@ csharp_EXTRA_DIST=                                                           \
   csharp/src/Google.Protobuf.Test/FieldCodecTest.cs                          \
   csharp/src/Google.Protobuf.Test/FieldMaskTreeTest.cs                       \
   csharp/src/Google.Protobuf.Test/GeneratedMessageTest.cs                    \
+  csharp/src/Google.Protobuf.Test/GeneratedMessageTest.Proto2.cs             \
   csharp/src/Google.Protobuf.Test/Google.Protobuf.Test.csproj                \
   csharp/src/Google.Protobuf.Test/IssuesTest.cs                              \
   csharp/src/Google.Protobuf.Test/JsonFormatterTest.cs                       \