Don't use static to mark constexpr variables as link lokal

[autoantwort]

Otherwise it is impossible to export types that depend on the strong_type since some of the used templated values are link lokal. Error message from gcc:

out/build/x64-debug-linux/vcpkg_installed/x64-linux/include/strong_type/type.hpp:235:23: note: 'template<class ...> constexpr const bool strong::impl::always_false< <template-parameter-1-1>...>' declared with internal linkage
  235 | static constexpr bool always_false = false;
      |                       ^~~~~~~~~~~~

for something like

module;

#include <strong_type/strong_type.hpp>

export module test;

import std;

using myint = strong::type<int, struct my_int_>;

with gcc 15.2