I added #include <dtostrf.h> to my code for Arduino DUE but no work. Then I search a solution from C/C++ text book that is about write your function and including.I was found the core folder and seem missing dtostrf.h. So i trial re-add the file and compiled...........OK :P.
That have been solved by adding header file #include <avr/dtostrf.h> to WString.h file.
Path: \arduino-1.5.6-r2\hardware\arduino\sam\cores\arduino