[libc++] Granularize <concept> includes

Reviewed By: ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D137283

NOKEYCHECK=True
GitOrigin-RevId: 89b356f05ab7aa3d96fc7b68aece6e7a5bdb0db5
diff --git a/include/valarray b/include/valarray
index f28e471..6c33d05 100644
--- a/include/valarray
+++ b/include/valarray
@@ -4931,6 +4931,7 @@
 
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
 #  include <algorithm>
+#  include <concepts>
 #  include <functional>
 #endif