[libc++][NFC] Fix incorrect include guard

Author: ldionne

libcxx/include/__memory/allocator_arg_t.h

@@ -7,8 +7,8 @@
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef _LIBCPP___FUNCTIONAL_ALLOCATOR_ARG_T_H
-#define _LIBCPP___FUNCTIONAL_ALLOCATOR_ARG_T_H
+#ifndef _LIBCPP___MEMORY_ALLOCATOR_ARG_T_H
+#define _LIBCPP___MEMORY_ALLOCATOR_ARG_T_H
 
 #include <__config>
 #include <__memory/uses_allocator.h>
@@ -72,4 +72,4 @@ __user_alloc_construct_impl(integral_constant<int, 2>, _Tp* __storage, const _Al
 
 _LIBCPP_END_NAMESPACE_STD
 
-#endif // _LIBCPP___FUNCTIONAL_ALLOCATOR_ARG_T_H
+#endif // _LIBCPP___MEMORY_ALLOCATOR_ARG_T_H