enum_dir: restore move semantics

This commit mostly reverts a previous commit:

commit b7bc687

    Avoid compiler warning with gcc by not using move semantics

The previous commit changed the semantics of the client
callback to use copy instead of move semantics on the
filename string to placate a compiler warning which was
later determined to be a false positive.

We revert to calling the client-provided func()
with move semantics on the filename parameter.

We also retain the use of std::invoke to call the
client-provided callback.

Signed-off-by: James Yonan <james@openvpn.net>

openvpn/common/enumdir.hpp

@@ -30,6 +30,7 @@
 #include <utility>
 #include <memory>
 #include <algorithm>
+#include <functional>
 
 #include <openvpn/common/size.hpp>
 #include <openvpn/common/exception.hpp>
@@ -52,7 +53,7 @@ inline bool enum_dir(const std::string &dirname,
     {
         std::string fn(e->d_name);
         if (fn != "." && fn != "..")
-            std::invoke(func, fn);
+            std::invoke(func, std::move(fn));
     }
     return true;
 }