Add missing header guard for Interface.hh

[chapulina]

🦟 Bug fix

Summary

Noticed it was missing during #248

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[methylDragon]

CI error

libcurl: (51) SSL: no alternative certificate subject name matches target host name 'staging-fuel.ignitionrobotics.org'
  [Wrn] [FuelClient.cc:309] Failed to fetch models from server, returning cached models.
  URL: https://staging-fuel.ignitionrobotics.org/
  Version: 1.0
  API key:

[mjcarroll]

CI error

I think this is due to changes in fuel. In the future, it would be nice to decouple a lot of these tests from having a fuel server actually running.