[gn build] Port 639a098

Author: llvmgnsyncbot

llvm/utils/gn/secondary/libcxx/include/BUILD.gn

@@ -582,6 +582,7 @@ if (current_toolchain == default_toolchain) {
       "__mdspan/extents.h",
       "__mdspan/layout_left.h",
       "__mdspan/layout_right.h",
+      "__mdspan/layout_stride.h",
       "__mdspan/mdspan.h",
       "__memory/addressof.h",
       "__memory/align.h",