misc/cgo/test: fix a gcc warning

See https://code.google.com/p/go/issues/detail?id=5227#c16.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/8833044

misc/cgo/test/issue5227.go

@@ -17,7 +17,7 @@ Fontinfo SansTypeface;
 extern void init();
 
 Fontinfo loadfont() {
-        Fontinfo f;
+        Fontinfo f = {0};
         return f;
 }