radvd: fix high jobcount build failure

Fix build failure:
http://autobuild.buildroot.org/results/48aabbc41dff2e4196dfd6e1e942cf6dbe050deb/build-end.log


because of a typo/missed update in the Makefile

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>