iputils has moved from the master tree to the packages feed, and is
switching from the abandoned skbuff.net upstream to
github.com/iputils/iputils.
Ref: https://git.openwrt.org/556698cedf9e86a0ffe9f148d4e8e733676c26f6
Signed-off-by: Petr Štetiar <ynezz@true.cz>