x/net/html/charset: fix nacl build
x/net/html/charset: fix nacl build

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/177880043

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