ipv4: fix build on linux/arm64
ipv4: fix build on linux/arm64

Change-Id: I16e3edd48a37ec385c24bd6de256ea896c8df652
Reviewed-on: https://go-review.googlesource.com/10094
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/net,raboof/net,mckinney-subgraph/net,cesanta/goxnet,cmars/tools,raboof/net,veliamsli/net,flowlo/go-net,cmars/tools,dafyddcrosby/net,zabawaba99/net,subuk/net,w4ngyi/net,subuk/net,albertjin/golang-net,muzining/net,muzining/net,yangxianjun/net,gomini/net,yangxianjun/net,mckinney-subgraph/net,albertjin/golang-net,kiip/net,zabawaba99/net,cmars/tools,DramaFever/golang.org-x-net,flowlo/go-net,cmars/tools,w4ngyi/net,kiip/net,gomini/net,cesanta/goxnet,dafyddcrosby/net,veliamsli/net,DramaFever/golang.org-x-net