Add zfs-load-key.sh to .gitignore

The generated zfs-load-key.sh file should have been added to
the .gitignore file as part of commit 7da8f8d8. And the
generated file should not be included in the repo.

Reviewed-by: Matthew Thode <prometheanfire@gentoo.org>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7134