AK: Enable format string warnings for AK printf wrappers

Author: https://github.com/ccapitalK
Commit: ccf4368ca5
Pull-request: https://github.com/SerenityOS/serenity/pull/4900

AK/StringBuilder.h

@@ -46,7 +46,7 @@ public:
     void append(char);
     void append_code_point(u32);
     void append(const char*, size_t);
-    void appendf(const char*, ...);
+    void appendf(const char*, ...) __attribute__((format(printf, 2, 3)));
     void appendvf(const char*, va_list);
 
     void append_escaped_for_json(const StringView&);