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 7da8f8d.  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

contrib/dracut/90zfs/.gitignore

@@ -3,5 +3,6 @@ module-setup.sh
 mount-zfs.sh
 parse-zfs.sh
 zfs-generator.sh
-zfs-needshutdown.sh
 zfs-lib.sh
+zfs-load-key.sh
+zfs-needshutdown.sh