kadm5: fix race in Makefile with kadm5_err.h

When running make with -j4, occasionally kadm5 fails due to a missing
header file kadm5_err.h. Fix the race condition.

Reported at https://bugzilla.redhat.com/1115164

Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Merge pull request #108 from ktdreyer/kadm5-make-race

kadm5: fix race in Makefile with kadm5_err.h