test: avoid undefined error in syntax/if.go.

Without this the gccgo version of errchk reports an unmatched
error.

R=golang-dev, rsc1
CC=golang-dev
https://golang.org/cl/4327041

test/syntax/if.go

@@ -6,6 +6,9 @@
 
 package main
 
+func x() {
+}
+
 func main() {
 	if {  // ERROR "missing condition"
 	}