package/libsidplay2: Fix build with gcc6

Fixes
http://autobuild.buildroot.net/results/787/787c715814b09d380ca521712f54cbcf5ae121d6/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>