chore(lp): fix compilation error

[collares]

/build/source/src/util/lp/permutation_matrix.h:126:44: error: cannot convert 'const std::vector<unsigned int>' to 'unsigned int*' in return
  126 |         unsigned * values() const { return m_permutation; }
      |                                            ^~~~~~~~~~~~~
      |                                            |
      |                                            const std::vector<unsigned int>

(cc @fpvandoorn, who I think was the last to maintain lean2)