test: correct type in declbad.go

The test is not about type mismatches and it causes
an extra error to appear.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6614062

test/declbad.go

@@ -41,7 +41,8 @@ func main() {
 	{
 		// multiline no new variables
 		i := f1
-		i := func() { // ERROR "redeclared|no new|incompatible"
+		i := func() int { // ERROR "redeclared|no new|incompatible"
+			return 0
 		}
 		_ = i
 	}