Add extra C# file to Makefile.am

Makefile.am

@@ -133,6 +133,7 @@ csharp_EXTRA_DIST=                                                           \
   csharp/src/Google.Protobuf/Collections/ProtobufEqualityComparers.cs        \
   csharp/src/Google.Protobuf/Collections/ReadOnlyDictionary.cs               \
   csharp/src/Google.Protobuf/Collections/RepeatedField.cs                    \
+  csharp/src/Google.Protobuf/Compatibility/MethodInfoExtensions.cs           \
   csharp/src/Google.Protobuf/Compatibility/PropertyInfoExtensions.cs         \
   csharp/src/Google.Protobuf/Compatibility/StreamExtensions.cs               \
   csharp/src/Google.Protobuf/Compatibility/TypeExtensions.cs                 \