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://golang.org/cl/69360043

src/pkg/runtime/mem_plan9.c

@@ -68,7 +68,7 @@ runtime·SysMap(void *v, uintptr nbytes, uint64 *stat)
 }
 
 void
-runtime·SysFault(void *v, uintptr n)
+runtime·SysFault(void *v, uintptr nbytes)
 {
 	USED(v, nbytes);
 }