ZTS: Fix devname2devid build on FreeBSD with libudev

When libudev is installed on FreeBSD, configure finds it and sets
WANT_DEVNAME2DEVID, but it isn't found by the linker because we
didn't specify where it is.

Use LIBUDEV_LIBS so the location of the library gets added to the
linker flags for devname2devid.
Also use LIBUDEV_CFLAGS here in case some other platform needs it.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10590