Add appropriate "EndProject" to solution file

In the latest merge a `EndProject` was missing in the solution.

Running `build.cmd Build` it failed with:
```
C:\GitD\akka.net\src\Akka.sln(450): Solution file error MSB5023: Error parsing the nested project 
section in solution file. A project with the GUID "{C1113300-74F1-446A-9914-3020C842EDF1}" is 
listed as being nested under project "{01167D3C-49C4-4CDE-9787-C176D139ACDD}", but 
does not exist in the solution.
```