iproute2: add missing libbpf dependency

iproute2: add missing libbpf dependency

This patch adds libbpf to the dependencies of tc-mod-iptables.

The package tc-mod-iptables is missing libbpf as a dependency,
which leads to the build failure described in bug #9491

    LIBBPF_FORCE=on set, but couldn't find a usable libbpf

The build dependency is already automatically added because some other
packages from iproute2 depend on libbpf, but bpftools has multiple build
variants. With multiple build variants none gets build by default and
the build system will not build bpftools before iproute2.

Fixes: #9491
Signed-off-by: Kien Truong <duckientruong@gmail.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>