Including atomic_pointer.h in port_posix

A recent CL (104348226) created the port_posix library, but omitted: port/atomic_pointer.h.

And when:

    [] test third_party/leveldb:all

was run this error was reported:

    //third_party/leveldb:port_posix does not depend on a
    module exporting 'third_party/leveldb/port/atomic_pointer.h'
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=105243399

port/port_posix.cc

@@ -7,7 +7,6 @@
 #include <cstdlib>
 #include <stdio.h>
 #include <string.h>
-#include "util/logging.h"
 
 namespace leveldb {
 namespace port {