package/unionfs: fix musl build

Fixes
http://autobuild.buildroot.net/results/020/020951a32ace3b4bca12a888eaaeb8b6c5633e4f/

[Peter: notice patch is now upstream:
 https://github.com/rpodgorny/unionfs-fuse/commit/f64b094eba93b042a1ed5964

]
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>