libpcap: fix canusb related build issue

Fixes http://autobuild.buildroot.net/results/1b57e479a643df11ea3b20f848085af4ef40799b



The libusb auto detection oddly enough succeeds, even though libusb isn't
available. Fix it by explicitly enabling/disabling canusb support.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>