Fix libSystem.dylib load on OS X 10.11 b3

Summary: Fix libSystem.dylib load on OS X 10.11 b3

Test Plan: Invoke `buck project ...` and make sure it works on OS X 10.11b3

(cherry picked from commit ccae1feb597b8449ae2bdea3a0170077aab10828)

programs/timing.py

diff --git a/programs/timing.py b/programs/timing.py
index 1fd0f7a..4f0b2b6 100644
--- a/programs/timing.py
+++ b/programs/timing.py
@@ -34,7 +34,7 @@
 elif platform.system() == 'Darwin':
     # From <mach/mach_time.h>
     KERN_SUCCESS = 0
-    libSystem = ctypes.CDLL('libSystem.dylib', use_errno=True)
+    libSystem = ctypes.CDLL('/usr/lib/libSystem.dylib', use_errno=True)
     mach_timebase_info = libSystem.mach_timebase_info
 
     class struct_mach_timebase_info(ctypes.Structure):