Restore the missing include in debian/rules.

[igorpeshansky]

Description

Restore the missing include in debian/rules. This undoes an accidental change from #1438, and makes the version string on Debian platforms use the codename again.

Related issue

b/340605738

How has this been tested?

Manual builds.
Before:

metric:{type:"agent.googleapis.com/agent/uptime" labels:{key:"version" value:"google-cloud-ops-agent-metrics/2.47.0-debian12"}}

After:

metric:{type:"agent.googleapis.com/agent/uptime" labels:{key:"version" value:"google-cloud-ops-agent-metrics/2.47.0-bookworm"}}

Checklist:

• Unit tests
  • Unit tests do not apply.
  • Unit tests have been added/modified and passed for this PR.
• Integration tests
  • Integration tests do not apply.
  • Integration tests have been added/modified and passed for this PR.
• Documentation
  • This PR introduces no user visible changes.
  • This PR introduces user visible changes and the corresponding documentation change has been made¹.
• Minor version bump
  • This PR introduces no new features.
  • This PR introduces new features, and there is a separate PR to bump the minor version since the last release already.
  • This PR bumps the version.

Footnotes

1. Note that this behavior was already undocumented, so no documentation changes were needed. ↩