systemlib scheduling_priorities: include the more generic px4_tasks.h

this makes sure the header can also be used under POSIX

src/modules/systemlib/scheduling_priorities.h

@@ -33,9 +33,7 @@
 
 #pragma once
 
-#ifndef __PX4_QURT
-#include <nuttx/sched.h>
-#endif
+#include <px4_tasks.h>
 
 /*      SCHED_PRIORITY_MAX    */
 #define SCHED_PRIORITY_FAST_DRIVER           SCHED_PRIORITY_MAX