Definition of struct timeout_cb get warning

In Xcode, The definition of struct timeout_cb in timeout.h
```c
struct timeout_cb {
	void (*fn)();
	void *arg;
}; /* struct timeout_cb */
```
In this line 
```c
void (*fn)();
```
 I get a warning "This function declaration is not a prototype Insert 'void'". Can you add a "void" as this:
```c
void (*fn)(void);
```