Fix installworld failure when kerberos source files have new timestamps

If a kerberos .hx source file is newer than the .h copy, but the content
is the same, then during buildworld the "cmp -s || cp" command in the
.hx.h rule would do nothing, leaving the .h copy with the older
timestamp.  During installworld the rule would again be invoked, causing
a failure as neither cmp or cp would exist in the temporary path.

As the underlying issue should be resolved by r262209, unconditionally
copy the file.

No objection:	peter@
Tested by:	gjb@
Sponsored by:	The FreeBSD Foundation