cmd/zpool cstyle issues

add missing headers.
usage() is no-return, so anything after call to it is unreachable code.
use (void) cast where we do ignore return value.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Toomas Soome <tsoome@me.com>
Closes #17885

Author: tsoome