Fix compiler warning(range-based for loop initialization statements are a C++20 extension

[faywong]

Fixes compiler error

bktree.hpp:569:12: warning: range-based for loop initialization statements are a C++20 extension [-Wc++20-extensions]
  569 |       for (bool first = true; auto const &[_, node] : m_root->m_children) {
      |            ^~~~~~~~~~~~~~~~~~

I confirm that my PR:

• complies with the contribution guidelines

Changes:

move the declaration out of for loop