Add ATTRIBUTE_FORMAT for _db_doprnt, it will catch problems

with DBUG_PRINT format specifiers

include/my_dbug.h

@@ -36,7 +36,8 @@ extern	void _db_enter_(const char *_func_,const char *_file_,uint _line_,
 extern	void _db_return_(uint _line_,const char **_sfunc_,const char **_sfile_,
 			 uint *_slevel_);
 extern	void _db_pargs_(uint _line_,const char *keyword);
-extern	void _db_doprnt_ _VARARGS((const char *format,...));
+extern	void _db_doprnt_ _VARARGS((const char *format,...))
+  ATTRIBUTE_FORMAT(printf, 1, 2);
 extern	void _db_dump_(uint _line_,const char *keyword,const char *memory,
 		       uint length);
 extern	void _db_output_(uint flag);