ftl-build: add libjournal

[darkexplosiveqwx]

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What does this PR aim to accomplish?:

Add https://github.com/darkexplosiveqwx/libjournal/ to the build container for the planned structured logging to journald.
Discussed in pi-hole/FTL#2897 (comment)

TODO: mirror onto https://ftl.pi-hole.net/libraries

Putting in draft mode for now, since I'd like to test a bit more and maybe need to bump the version. Already did some work last week and both the library and build system now seem functional.

How does this PR accomplish the above?:

Link documentation PRs if any are needed to support this PR:

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