mbuf: remove unneeded atomic generic header include

mbuf: remove unneeded atomic generic header include

There is no need for the direct inclusion of the generic/ header [1]
now that we don't use the rte_atomic API anymore.

It was the last case of direct inclusion of the generic/ headers,
so the flag -Wno-unused-function can be dropped.

1: https://git.dpdk.org/dpdk/commit/?id=3eb860b08eb7

Fixes: e41d27a68df6 ("mbuf: remove atomic reference counters")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>