Add attributions so printf like functions can have their arguments checked.

runtime/platform/json.h

Index: runtime/platform/json.h
diff --git a/runtime/platform/json.h b/runtime/platform/json.h
index ebbace9539eb52c348cc209705c9b8ecae917240..0da90b8e75c4f1db71ab4a34803f9d4949f378e8 100644
--- a/runtime/platform/json.h
+++ b/runtime/platform/json.h
@@ -116,7 +116,7 @@ class TextBuffer : ValueObject {
   explicit TextBuffer(intptr_t buf_size);
   ~TextBuffer();
 
-  intptr_t Printf(const char* format, ...);
+  intptr_t Printf(const char* format, ...) PRINTF_ATTRIBUTE(2, 3);
   void AddChar(char ch);
   void AddUTF8(uint32_t ch);
   void AddEscapedChar(uint32_t ch);