Try to appease clang

[ehughsbaird]

Summary

None

Purpose of change

Clang-tidy keeps making this suggestion in my PR: https://github.com/CleverRaven/Cataclysm-DDA/actions/runs/9634796206/job/26570985402?pr=74700#step:9:913

Error: /home/runner/work/Cataclysm-DDA/Cataclysm-DDA/src/map.cpp:5248:82: error: the parameter 'obj' is copied for each invocation but only used as a const reference; consider making it a const reference [performance-unnecessary-value-param,-warnings-as-errors]
 5248 | std::pair<item *, tripoint> map::_add_item_or_charges( const tripoint &pos, item obj,
      |                                                                                  ^
      |                                                                             const  &

This doesn't compile.

Describe the solution

Silence the clang-tidy check.

Testing

Watch the clang-tidy run on this PR.