package/keepalived: add optional libnftnl dependency

nftables support has been added since version 2.0.11 and
https://github.com/acassen/keepalived/commit/aa12175ea638912a7789e832634d7ccf80fdf48d



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>