zfs-tests: fix warnings when packaging some .shlib files

This change prevents the following warning when packaging some zfs-tests
files:

• WARNING: ./usr/src/zfs-0.8.0/tests/zfs-tests/include/zpool_script.shlib is executable but has empty or no shebang, removing executable bit

Reviewed by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #8787