package/binutils: bump to version 2.31.1

>From the release note:
  This is a point release over the previous 2.31 version, which was
  necessary as a source file was accidentally omitted from that
  release.

  This release also contains a fix for PR gold/23409 where the gold
  linker could end up creating duplicate copies of some symbols.

https://sourceware.org/ml/binutils/2018-07/msg00286.html



Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>