buildroot

keepalived: fix libnl optional dependency

List libnl as a dependency when it is enabled. Make sure that
libnfnetlink is actually enabled when listing it as a dependency.

Cc: Ilias Apalodimas <apalos@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>