Clean up gPTP IPC include paths and BUILD remapping

Follow-up for the discussion in [https://github.com/eclipse-score/time/pull/54]

The current PR keeps the scope limited to include guards only. As discussed, include paths such as:

#include "score/ts_client/src/gptp_ipc_channel.h"

should potentially be cleaned up to:

#include "score/ts_client/gptp_ipc_channel.h"

This may require adjusting strip_include_prefix / include_prefix in BUILD files and updating affected include sites consistently across the module.

Scope:
- Review existing include path conventions for score/ts_client
- Decide whether BUILD remapping should be adjusted
- Update include paths consistently if appropriate
- Keep this separate from the current include guard PR