eliminate mac xcode 5 only warning

on os x 10.9.2 unistd.h defines sysconf as

long	 sysconf(int);

R=mtklein@google.com, reed@google.com

Author: caryclark@google.com

Review URL: https://codereview.chromium.org/261673004

git-svn-id: http://skia.googlecode.com/svn/trunk@14559 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/utils/SkThreadPool.h b/include/utils/SkThreadPool.h
index 295b1b4..c99c5c4 100644
--- a/include/utils/SkThreadPool.h
+++ b/include/utils/SkThreadPool.h
@@ -26,7 +26,7 @@
     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