buildroot

lftp: indicate why autoreconf is needed

We no longer have any patch touching configure.ac/Makefile.am, so one
might wonder why we're autoreconfiguring the package. The reason is
that our libtool patch doesn't apply on the code provided by the lftp
tarball.

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