net/mlx5: use pkg-config to handle SUSE libmnl

net/mlx5: use pkg-config to handle SUSE libmnl

SUSE decided to install the libmnl include file in a non-standard
place: /usr/include/libmnl/libmnl/libmnl.h

This was probably a mistake by the SUSE package maintainer,
but hard to get fixed. Workaround the problem by pkg-config to find
the necessary include directive for libmnl.

Fixes: 20b71e92ef8e ("net/mlx5: lay groundwork for switch offloads")
Cc: stable@dpdk.org
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Yongseok Koh <yskoh@mellanox.com>
Acked-by: Luca Boccassi <bluca@debian.org>