package/openocd: select libusb when selecting libusb-compat

libusb-compat depends on libusb package, so we need to select
libusb package at the same time as libusb-compat to avoid an
unmet dependency.

Since 28f67899, the build
stop if a package build is trigged at Makefile level without
being selected at Config.in level.

This is the case here with BR2_PACKAGE_OPENOCD_USBPROG which
select BR2_PACKAGE_LIBUSB_COMPAT without selecting
BR2_PACKAGE_LIBUSB.

Fixes:
http://autobuild.buildroot.net/results/b4a/b4a8c54eed6bf75ab8ade0653953d19792155727/



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>