[gcc r16-444] libstdc++: Fix module std export for std::extents

[Jonathan Wakely]

https://gcc.gnu.org/g:0a5855d14dbe25c0890da5c5550013c795aab90e

commit r16-444-g0a5855d14dbe25c0890da5c5550013c795aab90e
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed May 7 11:54:24 2025 +0100

    libstdc++: Fix module std export for std::extents
    
    libstdc++-v3/ChangeLog:
    
            * src/c++23/std.cc.in: Fix export for std::extents.

Diff:
---
 libstdc++-v3/src/c++23/std.cc.in | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/libstdc++-v3/src/c++23/std.cc.in b/libstdc++-v3/src/c++23/std.cc.in
index 0df27cd7e7da..73316d168c94 100644
--- a/libstdc++-v3/src/c++23/std.cc.in
+++ b/libstdc++-v3/src/c++23/std.cc.in
@@ -1834,10 +1834,13 @@ export namespace std
 }
 
 // <mdspan>
+#if __glibcxx_mdspan
+export namespace std
 {
   using std::extents;
   // FIXME layout_*, default_accessor and mdspan
 }
+#endif
 
 // 20.2 <memory>
 export namespace std