[kernel] Fix build breakage

Three issues that caused build breaks when regenerating the build
directory after the previous commits:

- system.def was including endpoint.def
- syscalls/vm_area.cpp was including j6/signals.h
- util/util.h was missing an include of stddef.h

src/kernel/syscalls/vm_area.cpp

@@ -1,5 +1,4 @@
 #include <j6/errors.h>
-#include <j6/signals.h>
 #include <j6/types.h>
 
 #include "logger.h"