Fix LuaJIT flag in CMake configuration

[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

-DUSE_LUAJIT=ON is being used in the wrong step (build instead of configure) so the flag does not apply as expected.

It can be noticed in builds where CMake says

Unknown argument -DUSE_LUAJIT=on