arp: fix arp_vft callback
arp: fix arp_vft callback

arp proxy is not enabled correctly by ip4_neighbor_proxy_enable()

Type: fix
Signed-off-by: Mercury <481f9b581fa78632db783af3eaf67d5d12f628b9@gmail.com>
Change-Id: I672b5d4a9652030b5604e9d39743c39cb93a2531

FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp