hw/net/net_rx_pkt: Enforce alignment for eth_header

eth_strip_vlan and eth_strip_vlan_ex refers to ehdr_buf as struct
eth_header. Enforce alignment for the structure.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Sriram Yagnaraman <sriram.yagnaraman@est.tech>
Signed-off-by: Jason Wang <jasowang@redhat.com>