Add missing copyright

[0xnm]

What does this PR do?

Following a change made in #106 add a missing copyright to the new file added in #104.

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [ ] Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• [ ] Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

Merging #107 (2feb77d) into develop (e13be0d) will increase coverage by 0.02%. The diff coverage is n/a.

@@              Coverage Diff              @@
##             develop     #107      +/-   ##
=============================================
+ Coverage      88.43%   88.45%   +0.02%     
- Complexity       162      163       +1     
=============================================
  Files             21       21              
  Lines            510      511       +1     
  Branches          61       61              
=============================================
+ Hits             451      452       +1     
  Misses            38       38              
  Partials          21       21              

Impacted Files	Coverage Δ
...otlin/com/datadog/gradle/plugin/internal/ApiKey.kt	100.00% <ø> (ø)