[NaCl SDK] Upate atomic ops in nacl_io

native_client_sdk/src/libraries/sdk_util/library.dsc

 {
   'TOOLS': ['newlib', 'glibc', 'pnacl', 'win'],
   'SEARCH': [
     '.'
   ],
   'TARGETS': [
     {
       'NAME' : 'sdk_util',
       'TYPE' : 'lib',
       'SOURCES' : [
         'thread_pool.cc'
       ]
     }
   ],
   'HEADERS': [
     {
       'FILES': [
         'atomicops.h',
         'auto_lock.h',
         'macros.h',
         'ref_object.h',
         'scoped_ref.h',
+        'simple_lock.h',
         'thread_pool.h',
         'thread_safe_queue.h'
       ],
       'DEST': 'include/sdk_util',
     }
   ],
   'DEST': 'src',
   'NAME': 'sdk_util',
 }