Fix ArgumentError for Integrations::Campfire

What does this MR do and why?

Contributes to #393403 (closed)

Sentry error: https://sentry.gitlab.net/gitlab/gitlabcom/issues/3339218/

Problem

build_message method in Integrations::Campfire accidentally redefines the same method from Integrations::Loggable.

Solution

Rename the method

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