vhost: fix build dependency

vhost: fix build dependency

Commit d0cf91303d73 added dependency on librte_net headers to vhost
but did not add this to the Makefile, which makes builds
non-deterministic. Curiously it is non-parallel build that is
consistently broken by this missing dependency, usually it's the other
way around, but trying to build without -j(n) fails with:

	lib/librte_vhost/vhost_rxtx.c:41:20:
	fatal error: rte_ip.h: No such file or directory

Fixes: d0cf91303d73 ("vhost: add Tx offload capabilities")

Signed-off-by: Panu Matilainen <pmatilai@redhat.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>