Add an autogenerated .gitignore.

Generated with the following atrocity:

    { echo '# Autogenerated from svn:ignore' ; echo '*~';
      for dir in `find . -name .svn -prune , -false -o -type d ! -name .svn`;
        do svn pg svn:ignore $dir | sed -n "/./s|^|${dir#.}/|p";
      done; }  > ../../ocaml/.gitignore