package/iputils: fix IPUTILS_PERMISSIONS

package/iputils: fix IPUTILS_PERMISSIONS

The IPUTILS_PERMISSIONS variable was wrong, because iputils installs
its programs in /usr/bin, not in /bin. It probably was only tested
with the merged /usr option enabled.

This commit fixes IPUTILS_PERMISSIONS, taking into account the change
done in 6e18892733 "package/iputils: move binaries to the location also
used by Busybox":

 - arping is moved by the post-install target hook to /usr/sbin

 - clockdiff is installed by iputils in /usr/bin

 - ping is moved by the post-install target hook to /bin, which makes
   the previously incorrect path in IPUTILS_PERMISSIONS now correct

Fixes:

  http://autobuild.buildroot.net/results/fe378bca29c86a681ba9ad40386cb89248195c50/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>