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

native_client_sdk/src/examples/demo/voronoi/example.dsc

 {
   'TOOLS': ['newlib', 'glibc', 'pnacl'],
   'TARGETS': [
     {
       'NAME' : 'voronoi',
       'TYPE' : 'main',
       'SOURCES' : [
-        'voronoi.cc',
-        'threadpool.cc',
-        'threadpool.h'
+        'voronoi.cc'
       ],
 
-      'LIBS': ['ppapi_cpp', 'ppapi', 'pthread']
+      'LIBS': ['sdk_util', 'ppapi_cpp', 'ppapi', 'pthread']
     }
   ],
   'DATA': [
     'example.js',
   ],
   'DEST': 'examples/demo',
   'NAME': 'voronoi',
   'TITLE': 'Multi-Threaded Voronoi Demo',
   'GROUP': 'Demo'
 }