[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 {