[gcc r16-7716] libstdc++: add using std::size_t

[Torbjorn Svensson]

https://gcc.gnu.org/g:1c842aa818b3199677e102bc28adfcfccf6d1bb0

commit r16-7716-g1c842aa818b3199677e102bc28adfcfccf6d1bb0
Author: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>
Date:   Thu Feb 26 12:04:12 2026 +0100

    libstdc++: add using std::size_t
    
    Without a definition for size_t in global namespace, there are errors
    like this one for arm-none-eabi:
    
    .../requirements_neg.cc:5: error: 'size_t' has not been declared
    
    libstdc++-v3/ChangeLog:
    
            * testsuite/29_atomics/atomic_ref/requirements_neg.cc: Add
            using std::size_t.
    
    Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>

Diff:
---
 libstdc++-v3/testsuite/29_atomics/atomic_ref/requirements_neg.cc | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_ref/requirements_neg.cc b/libstdc++-v3/testsuite/29_atomics/atomic_ref/requirements_neg.cc
index 8b0abbde023f..a3613e0e7336 100644
--- a/libstdc++-v3/testsuite/29_atomics/atomic_ref/requirements_neg.cc
+++ b/libstdc++-v3/testsuite/29_atomics/atomic_ref/requirements_neg.cc
@@ -2,6 +2,8 @@
 
 #include <atomic>
 
+using std::size_t;
+
 template<size_t N>
 struct NonTrivial
 {