package/gnupg2: add optional support for libusb-compat

When libusb-compat was compiled before, gnupg2 will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/libexec/scdaemon | grep NEEDED
[...]
 0x0000000000000001 (NEEDED)             Shared library: [libusb-0.1.so.4]
[...]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>