libffi: fix headers location for host-libffi

There is a special hook for target-libffi, that moves headers
from /usr/lib/libffi-version/include to /usr/include. This patch
adds the same procedure for host-libffi.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
[Thomas:
 - factorize code between the staging headers cleanup and the host
   headers cleanup.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>