package/quagga: add optional support for libcap

When libcap was compiled before, quagga will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/bin/vtysh | grep NEEDED | sort
 0x0000000000000001 (NEEDED)             Shared library: [libcap.so.2]
[...]

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