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

lib/libzutil/zutil_import.c

@@ -224,6 +224,7 @@ typedef struct vdev_dev_strs {
 	char	vds_devphys[128];
 } vdev_dev_strs_t;
 
+#ifdef HAVE_LIBUDEV
 /*
  * Obtain the persistent device id string (describes what)
  *
@@ -398,6 +399,7 @@ udev_device_is_ready(struct udev_device *dev)
 	return (udev_device_get_property_value(dev, "DEVLINKS") != NULL);
 #endif
 }
+#endif /* HAVE_LIBUDEV */
 
 /*
  * Wait up to timeout_ms for udev to set up the device node.  The device is