cmd/prof: don't build on Plan 9 for now.

R=golang-dev, bradfitz, akumar
CC=golang-dev
https://golang.org/cl/5629054

src/cmd/prof/main.c

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build !plan9
+
 #include <u.h>
 #include <time.h>
 #include <libc.h>