Change type of Printf's args to ... interface{}

R=rsc
CC=golang-dev
https://golang.org/cl/197043
diff --git a/test/bench/pidigits.go b/test/bench/pidigits.go
index 430c118..aaa9f53 100644
--- a/test/bench/pidigits.go
+++ b/test/bench/pidigits.go
@@ -92,7 +92,7 @@
 	bignum.Iscale(numer, 10)
 }
 
-func printf(s string, arg ...) {
+func printf(s string, arg ...interface{}) {
 	if !*silent {
 		fmt.Printf(s, arg)
 	}