Avoid compiler warning with gcc by not using move semantics

gcc 12+ warn about temporary that used after its lifetime when we use
the move semantics here. Since the code here is not super performance
critical just remove the move semantics to be able to compile with
Werror.

Signed-off-by: Arne Schwabe <arne@openvpn.net>

openvpn/common/enumdir.hpp

@@ -52,7 +52,7 @@ inline bool enum_dir(const std::string &dirname,
  {
  std::string fn(e->d_name);
  if (fn != "." && fn != "..")
- func(std::move(fn));
+ std::invoke(func, fn);
  }
  return true;
 }