Fix Makefile.am by adding generated files

Makefile.am

@@ -126,6 +126,8 @@ csharp_EXTRA_DIST=                                                           \
   csharp/src/Google.Protobuf.Test/TestCornerCases.cs                         \
   csharp/src/Google.Protobuf.Test/TestProtos/ForeignMessagePartial.cs        \
   csharp/src/Google.Protobuf.Test/TestProtos/MapUnittestProto3.cs            \
+  csharp/src/Google.Protobuf.Test/TestProtos/OldExtensions1.cs               \
+  csharp/src/Google.Protobuf.Test/TestProtos/OldExtensions2.cs               \
   csharp/src/Google.Protobuf.Test/TestProtos/TestMessagesProto2.cs           \
   csharp/src/Google.Protobuf.Test/TestProtos/TestMessagesProto3.cs           \
   csharp/src/Google.Protobuf.Test/TestProtos/UnittestCustomOptionsProto3.cs  \