Add IncludeSource in csproj as per review comments

csharp/src/Google.Protobuf/Google.Protobuf.csproj

@@ -18,6 +18,7 @@
     <RepositoryType>git</RepositoryType>
     <RepositoryUrl>https://github.com/nodatime/nodatime.git</RepositoryUrl>
     <IncludeSymbols>true</IncludeSymbols>
+    <IncludeSource>true</IncludeSource>
   </PropertyGroup>
 
   <!--