Add missing lock annotation to the tx inventory send buffer
Add missing lock annotation to the tx inventory send buffer

Summary:
This is guarded by the `cs_tx_inventory` mutex but is lacking the
annotation.

Test Plan:
With clang:
  ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9491

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