Fix timer_delta_us declaration

Fixes this:

```
$ make
cc test.c src/timer.c -o test -Wall -Wextra
src/timer.c:42:13: error: conflicting types for 'timer_delta_us'
suseconds_t timer_delta_us (timer_t* t)
            ^
src/timer.h:54:6: note: previous declaration is here
long timer_delta_us(timer_t* t);
     ^
1 error generated.
make: *** [test] Error 1
```