Restore .gitignore of local OPAM switches
Restore .gitignore of local OPAM switches

This restores

  d7fb5767d3796d030b254e03a3c750a11f5e3122

which was accidentally undone by

  d8b1e595de87ee5334b88bda93dc71d08b3bdbb7

[skip ci]

rneswold/lwt,c-cube/lwt,c-cube/lwt,ocsigen/lwt,ocsigen/lwt,rneswold/lwt,c-cube/lwt,ocsigen/lwt,rneswold/lwt