debian11: solve libsystemd-dev dependencies

[v1v]

See https://packages.debian.org/bullseye/libsystemd-dev

Closes #311

[mergify]

This pull request does not have a backport label. Could you fix it @v1v? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d is the label to automatically backport to the 1./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[elasticmachine]

💔 Build Failed

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• Start Time: 2023-08-21T09:19:06.581+0000

• Duration: 56 min 33 sec

Steps errors

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Shell Script

• Took 15 min 55 sec . View more details here
• Description: make -C go -f Makefile.debian11 build

Shell Script

• Took 14 min 18 sec . View more details here
• Description: make -C go -f Makefile.debian11 build

Shell Script

• Took 15 min 48 sec . View more details here
• Description: make -C go -f Makefile.debian11 build

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[v1v]

I fixed the original error, though there is another failure afterwards when building the arm docker image, see #311 (comment)

[ycombinator]

This fix would also help unblock #314 and, in turn, make the govulncheck check pass on all Elastic Agent PR builds. Thanks!

[v1v]

#316 is the one fixing it