Add .gitattributes to mark generated Rust files as generated

[jnbooth]

Per @redstrate's request, this PR adds a .gitattributes file to mark generated files as autogenerated. That way, when one of the generate.sh scripts in cxx-qt-lib runs, it will be clearly marked as generated in the corresponding PR:

[LeonMatthesKDAB]

Thank you for taking the time to contribute this @jnbooth .

Weird how Github calls it specifically linguist-generated, instead of just generated, but that seems to be the law of the land:
https://docs.github.com/en/repositories/working-with-files/managing-files/customizing-how-changed-files-appear-on-github

[LeonMatthesKDAB]

Ah, please note that like all other files in the repository, it needs a license header (as pointed out by CI).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (4bab308) to head (28c7120).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1150   +/-   ##
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  Coverage   100.00%   100.00%           
=========================================
  Files           71        71           
  Lines        11967     11967           
=========================================
  Hits         11967     11967           

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