[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

definitions/objects/system.def

@@ -1,6 +1,3 @@
-import "objects/endpoint.def"
-import "objects/vma.def"
-
 # The system object represents a handle to kernel functionality
 # needed by drivers and other priviledged services
 object system : object {

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"

src/libraries/util/util/util.h

@@ -2,6 +2,7 @@
 /// \file util.h
 /// Utility functions used in other util code
 
+#include <stddef.h>
 #include <stdint.h>
 
 namespace util {