Add missing import of annif.eval in MLLM backend

[osma]

The MLLM backend was missing an import of annif.eval. This didn't always cause issues because usually it was imported by other modules such as annif.cli. It may have caused tests to occasionally hang on GitHub Actions CI.

This PR simply adds the missing import.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Merging #562 (4d50840) into master (4fba412) will increase coverage by 4.46%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #562      +/-   ##
==========================================
+ Coverage   95.03%   99.50%   +4.46%     
==========================================
  Files          82       82              
  Lines        5443     5444       +1     
==========================================
+ Hits         5173     5417     +244     
+ Misses        270       27     -243     

Impacted Files	Coverage Δ
annif/backend/mllm.py	100.00% <100.00%> (ø)
tests/conftest.py	100.00% <0.00%> (+1.20%)	⬆️
annif/analyzer/__init__.py	100.00% <0.00%> (+3.22%)	⬆️
tests/test_project.py	100.00% <0.00%> (+7.34%)	⬆️
annif/backend/__init__.py	100.00% <0.00%> (+13.20%)	⬆️
tests/test_backend.py	100.00% <0.00%> (+15.78%)	⬆️
annif/backend/mixins.py	95.12% <0.00%> (+24.39%)	⬆️
tests/test_analyzer_spacy.py	100.00% <0.00%> (+80.00%)	⬆️
tests/test_backend_fasttext.py	100.00% <0.00%> (+92.94%)	⬆️
annif/backend/fasttext.py	100.00% <0.00%> (+93.47%)	⬆️
... and 1 more

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[osma]

Unfortunately this didn't fix the hanging tests, but it's still a problem worth fixing, so I'm merging this anyway.