libffi: remove some header files installed in /usr/lib in the target

For some reason, libffi installs some header files in
/usr/lib/libffi-<version>/include, which is a non-standard location
and therefore they do not get removed automatically by the
target-finalize logic. This commit adds a post-install hook in
libffi.mk to get rid of these unneeded headers on the target.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>