lftp: add missing host-pkgconf dependency

lftp configure.ac uses PKG_CHECK_MODULES, so it must have host-pkgconf
in its dependency, otherwise the autoreconf step fails due to the lack
of pkg.m4.

Fixes:

  http://autobuild.buildroot.org/results/b16/b16d964f275d4d8703665236b969cc19f7ef20be/



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