libkcapi: fix build with gcc 7.x

Add an upstream patch to make libkcapi build with gcc 7.x.

Fixes:

  http://autobuild.buildroot.net/results/01f/01f207ccd1cf5fc030510cb7a4eb2649e124b49b/



Signed-off-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>