pw_kvs: Add attributes on unused variables for debug
pw_kvs: Add attributes on unused variables for debug

Appends [[maybe_unused]] to some variables that are only used in DBG
statements to satisfy the static analyzer.

Change-Id: If692c6c6d102a0540ee12fc1811fe0136d715ada
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/31660
Reviewed-by: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Reviewed-by: Keir Mierle <69344103cc60b36d6e1fbdc31ce6504b2286c646@google.com>
Commit-Queue: Prashanth Swaminathan <7074f112da8ec89069105f3b18a4ce46a4722488@google.com>

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