libfdisk: fix cfdisk freespace analyze

The problem is how fdisk_partition_cmp_start() compare numbers, the
function returns result from "a->start - b->start", unfortunately the
numbers are uint64, but function returns "int".

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1170191
Signed-off-by: Karel Zak <kzak@redhat.com>