Fix type for filename in load*XML functions

[ArturKnopik]

Issues addressed:
Fixes #4347

Pull Request Prelude

• I have followed [proper The Forgotten Server code styling][code].
• I have read and understood the [contribution guidelines][cont] before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

[MillhioreBT]

This only happens on old compilers, I have this error too, but I keep this change local and nothing happens.
TFS is not to blame for us having dinosaurs

[ArturKnopik]

vs 19 is not that old :D

[MillhioreBT]

vs 19 is not that old :D

I thought the same thing. It would be great if this solution could be applied temporarily for those of us who still use older compilers. This way, it is more comfortable to test changes without having to make corrections and then remove them to make them compatible with master.

[ghost]

if this doesn't affect anything else, I see no problem in implementing

[ranisalt]

vs 19 is not that old :D

It's not the age, it's because the implementation of the standard library is super quirky. For example, the iterators in std::array<T, N> are not simply T* as they are in gcc/clang, Microsoft implements something else that isn't compatible.

That caused some issues in #4043, since fixed, but it's annoying to maintain unexpected things :(