AP_HAL: Scheduler: reorder includes

Follow the following order for includes:

   - Corresponding header file (if exists)
   - System headers
   - Other ArduPilot library headers
   - "Local" headers (from the same library)

libraries/AP_HAL/Scheduler.h

@@ -2,12 +2,14 @@
 #ifndef __AP_HAL_SCHEDULER_H__
 #define __AP_HAL_SCHEDULER_H__
 
-#include "AP_HAL_Namespace.h"
-#include "AP_HAL_Boards.h"
-
 #include <stdint.h>
+
 #include <AP_Progmem/AP_Progmem.h>
 
+#include "AP_HAL_Boards.h"
+#include "AP_HAL_Namespace.h"
+
+
 class AP_HAL::Scheduler {
 public:
     Scheduler() {}