gdb: Free inferior->priv when inferior exits

gdb: Free inferior->priv when inferior exits

(Where "exits" includes being killed or detached.)

Nothing is clearing inferior->priv currently.  This is a problem if we
change the inferior's process_stratum targets in a single debug
session.  This field is currently only used by darwin-nat.c, but a
follow up patch will make remote.c use it too.  Without the fix,
remote.c might end up mistaking the priv object allocated by
darwin-nat.c with its own.

(Found by inspection.)

gdb/ChangeLog:
2016-10-26  Pedro Alves  <palves@redhat.com>

	* inferior.c (exit_inferior_1): Free 'priv'.