sched: split sched.h into separate header files

kernel/object.c

@@ -1,6 +1,8 @@
 #include <kernel/locks.h>
 #include <kernel/object.h>
 #include <kernel/queue.h>
+#include <kernel/sched.h>
+#include <kernel/thread.h>
 
 #define HAS_OP(obj, opname) ((obj)->ob_type->ob_ops.opname)