Add missing <utility> include

After upgrading g++ package to 12.1.0 on archlinux I see the following
compilation error:

  /usr/include/argparse/argparse.hpp: In member function ‘void argparse::ArgumentParser::index_argument(list_iterator)’:
  /usr/include/argparse/argparse.hpp:1167:34: error: ‘as_const’ is not a member of ‘std’; did you mean ‘is_const’?
   1167 |     for (const auto &name : std::as_const(it->m_names)) {
        |                                  ^~~~~~~~
        |                                  is_const

It turns out that std::as_const comes from <utility> header [1] which
was not explicitly included.

[1] https://en.cppreference.com/w/cpp/utility/as_const

include/argparse/argparse.hpp

@@ -48,6 +48,7 @@ SOFTWARE.
 #include <string_view>
 #include <tuple>
 #include <type_traits>
+#include <utility>
 #include <variant>
 #include <vector>