ZTS: Move cksum to common system commands

The cksum command is used by delegate tests. We have it on FreeBSD,
so it should not have been moved to the Linux commands list.

Move it back to the common commands list.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10007

tests/zfs-tests/include/commands.cfg

@@ -19,6 +19,7 @@ export SYSTEM_FILES_COMMON='arp
     chgrp
     chmod
     chown
+    cksum
     cmp
     cp
     cpio
@@ -136,7 +137,6 @@ export SYSTEM_FILES_LINUX='attr
     blkid
     blockdev
     chattr
-    cksum
     dmidecode
     exportfs
     fallocate