Enable printf format argument checking.

The original attempt missed the fact that Print is a member function,
so the first argument is the implicit 'this'.

Change-Id: I963b668c5432804c767f0a2e3ef7dea5978a1218
diff --git a/ui.h b/ui.h
index 31a8a7f..a0580b7 100644
--- a/ui.h
+++ b/ui.h
@@ -63,7 +63,7 @@
 
     // Write a message to the on-screen log (shown if the user has
     // toggled on the text display).
-    virtual void Print(const char* fmt, ...) = 0; // __attribute__((format(printf, 1, 2))) = 0;
+    virtual void Print(const char* fmt, ...) __printflike(2, 3) = 0;
 
     // --- key handling ---