sched: split sched.h into separate header files

kernel/panic.c

@@ -2,7 +2,8 @@
 #include <kernel/libc/stdio.h>
 #include <kernel/machine/panic.h>
 #include <kernel/printk.h>
-#include <kernel/sched.h>
+#include <kernel/task.h>
+#include <kernel/thread.h>
 #include <stdarg.h>
 
 static int has_panicked = 0;