libusb: disable udev support for host variant

There's no guarantee that udev with development files is present in the
host, so disable it. Fixes:
http://autobuild.buildroot.net/results/7bb/7bbf6ec25250dea96503253ac468adfef77a9010/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>