Fix tar SELinux header

When comparing with files created by GNU tar, I noticed the following
difference:

61 RHT.security.selinux=unconfined_u:object_r:user_tmp_t:s0\x00\n
->
60 RHT.security.selinux=unconfined_u:object_r:user_tmp_t:s0\n

After making toybox tar match GNU one, it no longer complains about bad
header when trying to list the files inside.

Test: tar --selinux -cvf /tmp/a.tar /tmp/a
      tar --list -f /tmp/a.tar