Fix gcc errors (backport #477)

[mergify]

This is an automatic backport of pull request #477 done by Mergify.
Cherry-pick of 835054c has failed:

On branch mergify/bp/gvm-libs-21.04/pr-477
Your branch is up to date with 'origin/gvm-libs-21.04'.

You are currently cherry-picking commit 835054c.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   boreas/alivedetection.c

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md

Cherry-pick of c600697 has failed:

On branch mergify/bp/gvm-libs-21.04/pr-477
Your branch is ahead of 'origin/gvm-libs-21.04' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit c600697.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

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