runtime: fix plan9 build

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/12986043

src/pkg/runtime/mem_plan9.c

@@ -56,7 +56,7 @@ runtime·SysUnused(void *v, uintptr nbytes)
 }
 
 void
-runtime·SysUsed(void *v, uintptr n)
+runtime·SysUsed(void *v, uintptr nbytes)
 {
 	USED(v, nbytes);
 }