My last commit c1137b6db7 "libffi: fix
libffi.pc file" broke the host build as that still placed the header
files to /usr/lib/libffi-3.0.13/include/ and the libffi.pc file pointed
to /usr/include/ . With this patch I took the patch from Debian and
also made the install process to put the header files to /usr/include
in all situations.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>