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

R=golang-dev, bradfitz, akumar
CC=golang-dev
http://codereview.appspot.com/5629054
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go