eliminate mac xcode 5 only warning

include/utils/SkThreadPool.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkThreadPool_DEFINED
 #define SkThreadPool_DEFINED
 
 #include "SkCondVar.h"
 #include "SkRunnable.h"
 #include "SkTDArray.h"
 #include "SkTInternalLList.h"
 #include "SkThreadUtils.h"
 #include "SkTypes.h"
 
 #if defined(SK_BUILD_FOR_UNIX) || defined(SK_BUILD_FOR_MAC) || defined(SK_BUILD_FOR_ANDROID)
 #    include <unistd.h>
 #endif
 
 // Returns the number of cores on this machine.
 static inline int num_cores() {
 #if defined(SK_BUILD_FOR_WIN32)
     SYSTEM_INFO sysinfo;
     GetSystemInfo(&sysinfo);
     return sysinfo.dwNumberOfProcessors;
 #elif defined(SK_BUILD_FOR_UNIX) || defined(SK_BUILD_FOR_MAC) || defined(SK_BUILD_FOR_ANDROID)
-    return sysconf(_SC_NPROCESSORS_ONLN);
+    return (int) sysconf(_SC_NPROCESSORS_ONLN);
 #else
     return 1;
 #endif
 }
 
 template <typename T>
 class SkTThreadPool {
 public:
     /**
      * Create a threadpool with count threads, or one thread per core if kThreadPerCore.
@@ skipping 153 matching lines @@
         pool->fBusyThreads--;
         pool->fReady.unlock();
     }
 
     SkASSERT(false); // Unreachable.  The only exit happens when pool->fState is kHalting_State.
 }
 
 typedef SkTThreadPool<void> SkThreadPool;
 
 #endif