[chronik] use the node:: namespace for NodeContext

Summary: This unbreaks chronik after it was broken by D12763

Test Plan:

cmake -GNinja -DBUILD_BITCOIN_CHRONIK=1 ..
ninja

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12764