package/radvd: fix musl build

The autobuilders did not catch the error yet because they failed
earlier with other packages, but I am continuing the build using the
defconfig from:
http://autobuild.buildroot.net/results/6cc/6cc0f8c067e07deea688b9b97284601a596b898c/



[Thomas: replace with upstream patch directly, fetched from Github.]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>