Clean up various C source and headers to appease -Wstrict-prototypes

In C, func() is a function taking an unspecified number of arguments,
vs func(void) a function taking no arguments. In C++ both forms indicate
"no arguments."

Update these headers to use the (void) form, which is correct in both
languages and avoids complaints when -Wstrict-prototypes is specified.

src/rp2_common/hardware_timer/timer.c

@@ -92,7 +92,7 @@ static inline uint harware_alarm_irq_number(uint alarm_num) {
     return TIMER_IRQ_0 + alarm_num;
 }
 
-static void hardware_alarm_irq_handler() {
+static void hardware_alarm_irq_handler(void) {
     // Determine which timer this IRQ is for
     uint32_t ipsr;
     __asm volatile ("mrs %0, ipsr" : "=r" (ipsr)::);