[Services] Fix Clang errors with tunnelbroker

Summary:
Clang doesn't provide an alias for usize_t which
corresponds directly to amqp's overloaded types. So
explicity cast to a type which amqp has an overload for.

Test Plan: Follow test plan from D4766 on a mac device

Reviewers: atul, max, karol

Reviewed By: atul

Subscribers: ashoat, tomek, adrian, abosh

Differential Revision: https://phab.comm.dev/D4765