Add dependency of `generate_references` on `nmodl`.

[1uc]

This can be reproduced by:

cmake -B build && cmake --build build --target=generate_references

i.e. call --target=generate_references without building it first. It would complain about missing bin/nmodl.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.59%. Comparing base (98e6df2) to head (3dfd270).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1234 +/- ## ======================================= Coverage 86.59% 86.59% ======================================= Files 176 176 Lines 13053 13053 ======================================= Hits 11303 11303 Misses 1750 1750 ```

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[bbpbuildbot]

Logfiles from GitLab pipeline #203794 (:white_check_mark:) have been uploaded here!

Status and direct links:

• :white_check_mark: spack_setup
• :white_check_mark: build:intel
• :white_check_mark: build:nvhpc
• :white_check_mark: test:intel
• :white_check_mark: test:nvhpc