Why is itoa.h missing in the includes and thus disabling use of utoa() function?

I am using FQBN: stm32duino:STM32F1:genericSTM32F103C

**Using utoa(aNumber, tStringBuffer, 10); throws error: 'utoa' was not declared in this scope**
When I add a line `#include "itoa.h"` in wirish.h
```
#include <avr/interrupt.h>
#include "itoa.h"

#include <libmaple/stm32.h>
```
the error vanishes.

Pull request required?
