Move thread_pool.h into utils so it can be shared by more than one example.

native_client_sdk/src/libraries/nacl_io/library.dsc

 {
   'TOOLS': ['newlib', 'glibc', 'pnacl', 'win'],
   'SEARCH': [
     '.',
-    'pepper',
-    '../utils'
+    'pepper'
   ],
   'TARGETS': [
     {
       'NAME' : 'nacl_io',
       'TYPE' : 'lib',
       'SOURCES' : [
         "kernel_handle.cc",
         "kernel_intercept.cc",
         "kernel_object.cc",
         "kernel_proxy.cc",
@@ skipping 49 matching lines @@
       ],
       'DEST': 'include/nacl_io',
     },
     {
       'FILES': [
         "all_interfaces.h",
         "define_empty_macros.h",
         "undef_macros.h",
       ],
       'DEST': 'include/nacl_io/pepper',
-    },
-    {
-      'FILES': [
-        "auto_lock.h",
-        "macros.h",
-        "ref_object.h",
-        "thread_safe_queue.h"
-      ],
-      'DEST': 'include/utils',
     }
   ],
   'DEST': 'src',
   'NAME': 'nacl_io',
 }