sysutils/lockfile-progs: Fix build on releng/13

While building in on releng/13, the following error occurs:

lockfile-progs.c:312:7: error: expected expression

312 |       char *err = 0L;
    |       ^

lockfile-progs.c:313:26: error: use of undeclared identifier 'err'

313 |       int rc = asprintf(&err, "unrecognized error status (%d)", status);
    |                          ^

lockfile-progs.c:319:14: error: use of undeclared identifier 'err'

319 |       return err;
    |              ^

This small patch addresses this problem.

Approved by: yasu (maintainer)
Differential Revision: https://reviews.freebsd.org/D48037