libffi: minor corrections in post install hooks

In order to be sure that we just deal with the includes related with
the actual package described into libffi.mk it is better to explicitly
specify the version of the libffi we deal with into the post install
hook.
As dev files are deprecated in target rootfs target post install
hook was also deleted.

Signed-off-by: Nicolas Ménégale <nicolas.menegale@openwide.fr>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>