audio: liblc3: Fix global include path issue

-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

Signed-off-by: Frank Audun Kvamtrø <frank.kvamtro@nordicsemi.no>