[libc++] Mark standard-mandated includes as such

Reviewed By: ldionne, Mordante, #libc, saugustine

Spies: saugustine, MaskRay, arichardson, mstorsjo, jloser, libcxx-commits, arphaman

Differential Revision: https://reviews.llvm.org/D127953

NOKEYCHECK=True
GitOrigin-RevId: db1978b67431ca3462ad8935bf662c15750b8252
diff --git a/include/stack b/include/stack
index 7405896..59878c1 100644
--- a/include/stack
+++ b/include/stack
@@ -107,6 +107,10 @@
 #include <type_traits>
 #include <version>
 
+// standard-mandated includes
+#include <compare>
+#include <initializer_list>
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif