[dev.cc] cmd/gc: fix warning on Plan 9
[dev.cc] cmd/gc: fix warning on Plan 9

warning: src/cmd/gc/walk.c:1769 set and not used: on

LGTM=rsc
R=rsc, minux
CC=golang-codereviews
https://codereview.appspot.com/175850043

webfd/go-zh,d0f/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,bryanxu/go-zh,webfd/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh