package/libndp: fix musl build

Fixes
http://autobuild.buildroot.net/results/d42/d42bebe51bbec38f131840b6bbefdc162f0ad194/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>