ipv6: fix build on linux/arm64
ipv6: fix build on linux/arm64

Change-Id: I519fbce7681b18694336482e9135731647ed3efa
Reviewed-on: https://go-review.googlesource.com/10095
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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