sched: split sched.h into separate header files

sched/timer.c

@@ -1,7 +1,8 @@
-#include <kernel/sched.h>
-#include <kernel/printk.h>
-#include <kernel/cpu.h>
 #include <kernel/clock.h>
+#include <kernel/cpu.h>
+#include <kernel/printk.h>
+#include <kernel/sched.h>
+#include <kernel/thread.h>
 
 static void timeout_expiry(struct timer *timer)
 {