sched: split sched.h into separate header files

sched/core.c

@@ -4,6 +4,8 @@
 #include <kernel/object.h>
 #include <kernel/printk.h>
 #include <kernel/sched.h>
+#include <kernel/task.h>
+#include <kernel/thread.h>
 #include <kernel/vm-region.h>
 
 extern kern_status_t setup_kernel_task(void);