BLD Fix yet another meson.build dependency

[lesteve]

Another instance of #28820, issue seen in #29081 (comment).

FAILED: sklearn/metrics/_pairwise_distances_reduction/_datasets_pair.cpython-312-darwin.so.p/sklearn/metrics/_pairwise_distances_reduction/_datasets_pair.pyx.cpp
  cython -M --fast-fail -3 --cplus '-X language_level=3' '-X boundscheck=False' '-X wraparound=False' '-X initializedcheck=False' '-X nonecheck=False' '-X cdivision=True' '-X profile=False' --include-dir /Users/runner/work/1/s/build/cp312 sklearn/metrics/_pairwise_distances_reduction/_datasets_pair.pyx -o sklearn/metrics/_pairwise_distances_reduction/_datasets_pair.cpython-312-darwin.so.p/sklearn/metrics/_pairwise_distances_reduction/_datasets_pair.pyx.cpp

  Error compiling Cython file:
  ------------------------------------------------------------
  ...
  from ...utils._typedefs cimport float64_t, float32_t, int32_t, intp_t
  ^
  ------------------------------------------------------------

  sklearn/metrics/_pairwise_distances_reduction/_datasets_pair.pxd:1:0: relative cimport beyond main package is not allowed

The error happened because sklearn/metrics/__init__.py was not copied when calling cython.

The error can be reproduce locally on main with:

rm -rf my-build
meson setup my-build
ninja -C my-build sklearn/metrics/_pairwise_distances_reduction/_datasets_pair.cpython-312-x86_64-linux-gnu.so

[github-actions]

✔️ Linting Passed

All linting checks passed. Your pull request is in excellent shape! ☀️

_{Generated for commit: 6adba9c. Link to the linter CI: here}

[ogrisel]

LGTM.