[libc++][atomic_ref] add header to module map

Author: dalg24

libcxx/include/module.modulemap.in

@@ -1095,6 +1095,7 @@ module std_private_atomic_atomic_flag         [system] {
 }
 module std_private_atomic_atomic_init         [system] { header "__atomic/atomic_init.h" }
 module std_private_atomic_atomic_lock_free    [system] { header "__atomic/atomic_lock_free.h" }
+module std_private_atomic_atomic_ref          [system] { header "__atomic/atomic_ref.h" }
 module std_private_atomic_atomic_sync         [system] { header "__atomic/atomic_sync.h" }
 module std_private_atomic_check_memory_order  [system] { header "__atomic/check_memory_order.h" }
 module std_private_atomic_contention_t        [system] { header "__atomic/contention_t.h" }