Fix libudev dependency in libzutil

ZFS should be able to build without libudev installed. The recent
change for libzutil inadvertently broke that. Make the libudev code
conditional in zutil_import.c to resolve the build failure.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Don Brady <don.brady@delphix.com>
Closes #8097