[gn build] Port fc48765

Author: llvmgnsyncbot

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

@@ -554,6 +554,7 @@ if (current_toolchain == default_toolchain) {
       "__mdspan/extents.h",
       "__mdspan/layout_left.h",
       "__mdspan/layout_right.h",
+      "__mdspan/mdspan.h",
       "__memory/addressof.h",
       "__memory/align.h",
       "__memory/aligned_alloc.h",