Fix freebsd build.

BUG=2126
TEST=

Review URL: https://chromiumcodereview.appspot.com/10381149
Patch from Geoffrey Garside <ggarside@gmail.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11561 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/platform-freebsd.cc

@@ -554,6 +554,7 @@ class FreeBSDMutex : public Mutex {
     ASSERT(result == 0);
     result = pthread_mutex_init(&mutex_, &attrs);
     ASSERT(result == 0);
+    USE(result);
   }
 
   virtual ~FreeBSDMutex() { pthread_mutex_destroy(&mutex_); }