cmd/gc: moved usefield to correct section of go.h, from "reflect.c" to "walk.c".
cmd/gc: moved usefield to correct section of go.h, from "reflect.c" to "walk.c".

LGTM=iant
R=golang-codereviews, gobot, iant, dave
CC=golang-codereviews
https://codereview.appspot.com/108990044

Committer: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

webfd/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,webfd/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,d0f/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,bryanxu/go-zh