pkg/runtime/plan9: Warning remediation, for Plan 9 native.

. Missing declaration of runtime.brk_();
. Argument v in runtime.SysReserve() is not used;
  (I'd prefer a Plan 9-type solution...)

R=golang-dev, r, r2
CC=golang-dev
https://golang.org/cl/4368076

src/pkg/runtime/plan9/mem.c

@@ -4,6 +4,7 @@
 
 #include "runtime.h"
 #include "malloc.h"
+#include "os.h"
 
 extern byte end[];
 static byte *bloc = { end };
@@ -52,5 +53,6 @@ runtime·SysMap(void *v, uintptr nbytes)
 void*
 runtime·SysReserve(void *v, uintptr nbytes)
 {
+	USED(v);
 	return runtime·SysAlloc(nbytes);
 }