sched: split sched.h into separate header files

sched/workqueue.c

@@ -1,7 +1,8 @@
-#include <kernel/sched.h>
-#include <kernel/vm.h>
-#include <kernel/util.h>
 #include <kernel/cpu.h>
+#include <kernel/sched.h>
+#include <kernel/thread.h>
+#include <kernel/util.h>
+#include <kernel/vm.h>
 
 static struct worker_pool *__global_worker_pool = NULL;