package/sox: fix static build with id3tag

This build failure is raised since bump to
7524160b29a476f7e87bc14fddf12d349f9a3c5e

Fixes:
 - http://autobuild.buildroot.org/results/73efdacf237e3d567fa66f3b3f68e624f5e35bc7



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>