[services] Backup - Fix: add missing pragma once to headers

Summary:
Depends on D3485

This is more like a best practice, but it's pretty important and may lead to unpleasant errors

Test Plan:

cd services
yarn run-backup-service

Reviewers: palys-swm, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, palys-swm, Adrian, atul, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3779