runtime: fix build on Plan 9
runtime: fix build on Plan 9

warning: src/pkg/runtime/mem_plan9.c:72 param declared and not used: n
src/pkg/runtime/mem_plan9.c:73 name not declared: nbytes
src/pkg/runtime/mem_plan9.c:73 bad in naddr: NAME nbytes<>+0(SB)

LGTM=minux.ma, bradfitz
R=khr, minux.ma, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/69360043

wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old