iproute2: needs headers >= 3.0

The newest iproute2 version requires a toolchain with headers >= 3.0 for
proper namespace & bpf support, so adjust the dependencies and
dependants accordingly. Fixes:

http://autobuild.buildroot.net/results/fa8/fa810f310c7540e02c0ed84c4a3ce74ab602daa5/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>