Add missing pattern in TT.hs

Idris tripped over an incomplete pattern match in nextName' while typechecking
https://github.com/defanor/parcomb/blob/1cd15bf9c7e4a06c30ee5a804ea5d4d4993bbbab/VISD.idr

I traced and added the missing pattern match. The file typechecks and tests pass
but I have not tried to understand what is actually going on in TT.hs.