clang-tidy fixes for log services header

Silent clang-tidy warnings:

- remove unused header includes
- switch <systemd/sd_bus.h> to <systemd/sd-bus-protocol> for
  sd_bus_error usage
- change str.find usage to str.contains

Tested: Code compiles.

Change-Id: I814239d47e1586ebc28bccc00c08e41f711b9beb
Signed-off-by: Oliver Brewka <oliver.brewka@9elements.com>