package/audit: not available for musl

audit uses strndupa() which is missing from musl.

Even though the implementation of strndupa is not too complex, we won't
go as far as duplicating it in audit, and we just disable audit for the
musl C library.

Fixes;
    http://autobuild.buildroot.org/results/e22/e22a70f9ff14bc52f642a6135da44c14e41b6cbb/
    http://autobuild.buildroot.org/results/233/2333c9b3d8f81b15602263b918d422e440f09d60/


    ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Clayton Shotwell <clayton.shotwell@rockwellcollins.com>
Cc: Matthew Weber <matthew.weber@rockwellcollins.com>
Cc: Ryan Barnett <rjbarnet@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>