Ignore HPC files generated by GHC

"GHC creates a subdirectory .hpc in the current directory, and puts HPC index (.mix) files in there..."

https://www.haskell.org/ghc/docs/7.4.2/html/users_guide/hpc.html
Merge pull request #1223 from mlacorte/patch-1

Ignore HPC files generated by GHC