[Chore] Lint services/backup/CMakeLists.txt
Summary: Lint backup
Test Plan:
nix develop cmake-lint services/backup/CMakeLists.txt
Reviewers: atul, varun, max, karol
Reviewed By: atul
Subscribers: ashoat, tomek, adrian, abosh
Differential Revision: https://phab.comm.dev/D4565