libffi: enable the host build, needed for newer glib

Signed-off-by: Jonh Wendell <jonh.wendell@oiwifi.com.br>
Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/packages@34489 3c298f89-4303-0410-b956-a3cf2f4a3e73