add missing salt.utils import to modules.openbsdpkg

************* Module salt.modules.openbsdpkg
E0602: 37,23:list_pkgs: Undefined variable 'salt'
E0602: 39,7:list_pkgs: Undefined variable 'salt'
Merge pull request #5555 from cvrebert/patch-2

add missing salt.utils import to modules.openbsdpkg