stl/tuple: change uarg##N to internal linkage

8b6a9c2 (std::tuple support (Resolving #103) (#104), 2021-03-11) put
uarg##N in all translation units, which includes
boost/phoenix/stl/tuple.hpp or boost/phoenix/stl.hp, with external
linkage. Thus, we'll run into below error:

> multiple definition of `boost::phoenix::placeholders::uarg1'

Change it to internal linkage.

include/boost/phoenix/stl/tuple.hpp

@@ -110,7 +110,7 @@ namespace boost { namespace phoenix {
     namespace placeholders {
         #define BOOST_PP_LOCAL_LIMITS (1, BOOST_PHOENIX_ARG_LIMIT)
         #define BOOST_PP_LOCAL_MACRO(N)                                                \
-            auto uarg##N =                                                             \
+            const auto uarg##N =                                                       \
             boost::phoenix::get_<(N)-1>(boost::phoenix::placeholders::arg1);
         #include BOOST_PP_LOCAL_ITERATE()
     }