libffi: drop autoreconf

libffi: drop autoreconf

Trying to compile libffi now that automake has been updated to 1.17 will
fail, however there is no reason for us to be calling autoreconf as libffi
release tarballs already contain the generated configure script.

So, drop the unnecessary autoreconf that seems to be leftover from the
previous 3.4.2 bump as we dont have any patches targeting configure.ac nor
libffi in general.

Fixes: openwrt/openwrt#18041
Signed-off-by: Robert Marko <robimarko@gmail.com>