Fix the //sandbox/mac build when using a modern (10.7+) SDK.

sandbox/mac/xpc.h

Index: sandbox/mac/xpc.h
diff --git a/sandbox/mac/xpc.h b/sandbox/mac/xpc.h
index 5ea9f71990a0b44c0f680c39f51fca99e4ea757d..33d3945e387cf2116705dce227bead9393c24909 100644
--- a/sandbox/mac/xpc.h
+++ b/sandbox/mac/xpc.h
@@ -10,19 +10,20 @@
 #ifndef SANDBOX_MAC_XPC_H_
 #define SANDBOX_MAC_XPC_H_
 
+#include <AvailabilityMacros.h>
 #include <mach/mach.h>
 
 #include "sandbox/sandbox_export.h"
 
-// C++ library loader.
-#include "sandbox/mac/xpc_stubs.h"
-
 // Declares XPC object types. This includes <xpc/xpc.h> if available.
 #include "sandbox/mac/xpc_stubs_header.fragment"
 
 #if !defined(MAC_OS_X_VERSION_10_7) || \
     MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_X_VERSION_10_7
 
+// C++ library loader.
+#include "sandbox/mac/xpc_stubs.h"
+
 extern "C" {
 // Signatures for XPC public functions that are loaded by xpc_stubs.h.
 #include "sandbox/mac/xpc_stubs.sig"