package/kismet: add optional support for libcap

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

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

The build system offers no option to en-/disable libcap support:
https://www.kismetwireless.net/gitweb/?p=kismet.git;a=blob;f=configure.ac;h=e874b10f3c1d56e2deb5e8adf4b8da4066fdde56;hb=HEAD#l635



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