audio: liblc3: Fix global include path issue

[frkv]

-liblc3 added a library-internal path to zephyr_interface containing
a file that collides with common.h required e.g. by Mbed TLS
This commit fixes build-issues by making this include-folder
PRIVATE to the named library liblc3

[github-actions]

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[Thalley]

Looks correct to me

[github-actions]

Hi @frkv!
Congratulations on getting your very first Zephyr pull request merged 🎉🥳. This is a fantastic achievement, and we're thrilled to have you as part of our community!

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Thank you for your valuable input, and we look forward to seeing more of your contributions in the future! 🪁