package/netdata: add libcap optional dependency

libcap is an optional dependency which is enabled by default since the
addition of the package in commit
1d2bb469

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>