net/virtio: fix build with 0 headroom

net/virtio: fix build with 0 headroom

When using RTE_PKTMBUF_HEADROOM as 0, virito ethdev driver throws
compilation error
virtio_ethdev.c:1851:2: note: in expansion of macro ‘RTE_BUILD_BUG_ON’
RTE_BUILD_BUG_ON(RTE_PKTMBUF_HEADROOM
	< sizeof(struct virtio_net_hdr_mrg_rxbuf));

This patch change it into run-time check.

Bugzilla ID: 335
Fixes: dec08c28c0b3 ("virtio: check packet headroom at compile time")
Cc: stable@dpdk.org
Signed-off-by: Hemant Agrawal <hemant.agrawal@nxp.com>
Acked-by: Maxime Coquelin <maxime.coquelin@redhat.com>