package/dbus-broker: audit support needs libcap-ng

Since v14 in 2018, audit support has needed libcap-ng.

Fixes:
    http://autobuild.buildroot.org/results/43abdb85cc2f386d427cec1cfa876e20e3509cb8/



Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Norbert Lange <nolange79@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>