You have to include Microsoft.Owin.Host.HttpListener.dll in your project references.
You can add it through NuGet.
However, if the code executing:
WebApp.Start<SrvcHst> (...);
is contained within a class library, make sure that the executable consuming the library also includes the reference to Microsoft.Owin.Host.HttpListener.dll, or else it would not get deployed with your program, as there are no explicit references to it from the class library.
Have a look at your bin/Debug folder and make sure the DLL is there.