This makes null_ptid and minus_one_ptid "const". I think this is an
improvement because it means they can't be accidentally modified.
2019-03-12 Tom Tromey <tromey@adacore.com>
* common/ptid.c (null_ptid, minus_one_ptid): Now const.
* common/ptid.h (null_ptid, minus_one_ptid): Now const.