[libj6] Move remaining j6 headers out of src/include

This means the kernel now depends on libj6. I've added the macro
definition __j6kernel when building for the kernel target, so I can
remove parts with #ifdefs.

src/kernel/scheduler.cpp

@@ -1,6 +1,5 @@
 #include <stddef.h>
 
-#include <j6/init.h>
 #include <util/spinlock.h>
 
 #include "apic.h"