Make memory_order_acquire visible in global namespace

We were missing that using directive when including <atomic>.

Bug:17736764

Change-Id: I93673dc60cca47195ff7bd651840c77255fb2d4a

ndk/platforms/android-21/include/stdatomic.h

@@ -89,6 +89,7 @@ using std::atomic_signal_fence;
 using std::memory_order;
 using std::memory_order_relaxed;
 using std::memory_order_consume;
+using std::memory_order_acquire;
 using std::memory_order_release;
 using std::memory_order_acq_rel;
 using std::memory_order_seq_cst;