Add libiconv for identity service build
Summary:
Crate was added which links against it.
Depends on D3902
Test Plan:
nix develop && cd services/identity && cargo build && echo Success Reviewers: varun, ashoat Reviewed By: varun, ashoat Subscribers: ashoat, palys-swm, Adrian, atul, yayabosh, varun Differential Revision: https://phab.comm.dev/D4181