cmd/go: add missing newline at end of error message

Change-Id: I9277e869e171d9e051ff9a70ac578b8127fa74f8
Reviewed-on: https://go-review.googlesource.com/33897
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/build.go

@@ -3773,7 +3773,7 @@ func instrumentInit() {
 		return
 	}
 	if buildRace && buildMSan {
-		fmt.Fprintf(os.Stderr, "go %s: may not use -race and -msan simultaneously", flag.Args()[0])
+		fmt.Fprintf(os.Stderr, "go %s: may not use -race and -msan simultaneously\n", flag.Args()[0])
 		os.Exit(2)
 	}
 	if goarch != "amd64" || goos != "linux" && goos != "freebsd" && goos != "darwin" && goos != "windows" {