Use size_t in CWallet::MarkDestinationsDirty
Use size_t in CWallet::MarkDestinationsDirty

Summary: As per title.

Test Plan:
  ninja all check

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7835

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