fix(iomapserialize): add class Thing forward declaration

[ranisalt]

Pull Request Prelude

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

Changes Proposed

This missing forward declaration causes a build error when unity build is disabled:

/home/rsa/Documents/forgottenserver/src/iomapserialize.h:30:54: error: ‘Thing’ has not been declared
   30 |         static bool loadItem(PropStream& propStream, Thing* parent);
      |                                                      ^~~~~

It used to be Cylinder but it was changed in #5058 and the forward declaration wasn't added accordingly.

Issues addressed:

How to test:

[Copilot]

Pull request overview

This PR fixes a build error that occurs when unity build is disabled by adding the missing forward declaration for the Thing class. The parameter type was changed from Cylinder to Thing in a previous PR (#5058), but the corresponding forward declaration was not updated at that time.

• Adds class Thing; forward declaration to resolve compilation error
• Maintains alphabetical ordering of forward declarations

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