Remove unneeded header from libzpool/kernel.c

The sys/signal.h header doesn't exist on FreeBSD, nor is
it needed on Linux.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9510