[LIB:ARBITER] Addendum to 374f2b4d: Fix build.

- No need to use a PCH when compiling a 1-file library.
- Add the _NTSYSTEM_ definition.

- Since the arbiter.h file is to be included by the users of this
  library, namely only kernel-mode NT components (incl. NTOS), there
  is no need to include the standard kernel-mode headers in it.
  Include these headers instead in the private implementation of the
  library, arbiter.c.

sdk/lib/drivers/arbiter/arbiter.h

@@ -9,11 +9,6 @@
 #ifndef _ARBITER_H
 #define _ARBITER_H
 
-#ifndef _NTOSKRNL_
-#include <ntifs.h>
-#include <ndk/rtlfuncs.h>
-#endif
-
 typedef struct _ARBITER_ORDERING
 {
     ULONGLONG Start;