[version.syn] Add missing "freestanding" to feature-test macro

A misapplication of P2781R9.

Author: tkoeppe

source/support.tex

@@ -618,7 +618,7 @@
 #define @\defnlibxname{cpp_lib_complex_udls}@                      201309L // also in \libheader{complex}
 #define @\defnlibxname{cpp_lib_concepts}@                          202207L
   // freestanding, also in \libheader{concepts}, \libheader{compare}
-#define @\defnlibxname{cpp_lib_constant_wrapper}@                  202506L // also in \libheader{type_traits}
+#define @\defnlibxname{cpp_lib_constant_wrapper}@                  202506L // freestanding, also in \libheader{type_traits}
 #define @\defnlibxname{cpp_lib_constexpr_algorithms}@              202306L // also in \libheader{algorithm}, \libheader{utility}
 #define @\defnlibxname{cpp_lib_constexpr_atomic}@                  202411L // also in \libheader{atomic}
 #define @\defnlibxname{cpp_lib_constexpr_bitset}@                  202207L // also in \libheader{bitset}