Include hardware/timer.h (#90)

If you include sleep.h first you get a compile error as
hardware_alarm_callback_t is not defined. Fix this by including the
appropriate header.

Fixes #89

Author: peterharperuk

src/rp2_common/pico_sleep/include/pico/sleep.h

@@ -9,7 +9,7 @@
 
 #include "pico.h"
 #include "hardware/rosc.h"
-
+#include "hardware/timer.h"
 #include "pico/aon_timer.h"
 
 #ifdef __cplusplus