Fixup (lib): Missing feature test macro

Author: tkoeppe

source/support.tex

@@ -791,6 +791,7 @@
 #define @\defnlibxname{cpp_lib_rcu}@                               202306L // also in \libheader{rcu}
 #define @\defnlibxname{cpp_lib_reference_from_temporary}@          202202L // freestanding, also in \libheader{type_traits}
 #define @\defnlibxname{cpp_lib_reference_wrapper}@                 202403L // freestanding, also in \libheader{functional}
+#define @\defnlibxname{cpp_lib_reflection}@                        202506L // also in \libheader{meta}
 #define @\defnlibxname{cpp_lib_remove_cvref}@                      201711L // freestanding, also in \libheader{type_traits}
 #define @\defnlibxname{cpp_lib_result_of_sfinae}@                  201210L
   // freestanding, also in \libheader{functional}, \libheader{type_traits}