sched: split sched.h into separate header files

include/kernel/ringbuffer.h

@@ -1,8 +1,10 @@
 #ifndef KERNEL_RINGBUFFER_H_
 #define KERNEL_RINGBUFFER_H_
 
+#include <kernel/flags.h>
 #include <kernel/locks.h>
-#include <kernel/sched.h>
+#include <kernel/types.h>
+#include <kernel/wait.h>
 
 struct ringbuffer {
 	unsigned char *r_buffer;