[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/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 {