[package] libusb: prevent building unused stuff

[package] libusb: prevent building unused stuff

This patch prevents the processing of the 'doc' and 'tests'
subdirectories of the libusb package. Since the result of building these
directories is never included in the ipkg package, omitting them saves
compilation time and avoids possible errors if building the docs fails
because of some openjade error (as it happened in my case).

This is my first contribution to the list, so please tell me if
something's wrong or if I could do smth better next time :-)

Signed-off-by: Joachim Schlipper <jschlipper@gmx.de>
git-svn-id: svn://svn.openwrt.org/openwrt/packages@25436 3c298f89-4303-0410-b956-a3cf2f4a3e73