Add a comma before ellipsis in constexpr_swap.

gcc 15 complains that the comma is required before vararg ellipsis.

Fixes https://github.com/boostorg/integer/issues/35.

include/boost/integer/common_factor_rt.hpp

@@ -64,7 +64,7 @@ namespace boost {
             return a.swap(b);
          }
          template <class T, class U>
-         inline constexpr void constexpr_swap(T& a, U& b...) BOOST_GCD_NOEXCEPT(T)
+         inline constexpr void constexpr_swap(T& a, U& b, ...) BOOST_GCD_NOEXCEPT(T)
          {
             T t(static_cast<T&&>(a));
             a = static_cast<T&&>(b);