package/iputils: bump version to 31d947cf7156cf78d3f57e0bd82b33e6f6ece6b4

Includes a fix for static builds using libgcrypt.

Fixes:
 http://autobuild.buildroot.net/results/55a7db5c0cbc34bbf6a2cbc52713d8822deec5b2
 http://autobuild.buildroot.net/results/864d8901eb901182ce186cf9a845636040e242bb
 http://autobuild.buildroot.net/results/aba522aa164267993fb1f1bbc57733be0cbea680



Signed-off-by: Martin Bark <martin@barkynet.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>