dhcp: add RDEPENDS for dhcp-client

Add iproute2 RDEPENDS for dhcp-client as
/sbin/dhclient-systemd-wrapper which called
by dhclient.service depends on ip command which
provided by iproute2 package when systemd enabled
in DISTRO_FEATURES.

Signed-off-by: Mingli Yu <mingli.yu@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>