sched: split sched.h into separate header files

init/main.c

@@ -13,7 +13,9 @@
 #include <kernel/port.h>
 #include <kernel/printk.h>
 #include <kernel/sched.h>
+#include <kernel/task.h>
 #include <kernel/test.h>
+#include <kernel/thread.h>
 #include <kernel/vm-object.h>
 #include <stdint.h>