gpt: don't misbehave with e.g., a 9-entry partition array

gpt: don't misbehave with e.g., a 9-entry partition array

* libparted/labels/gpt.c (_generate_header): Correct size of
partition array entries to round up to nearest multiple of
sector size, so that we set gpt->PartitionEntryLBA correctly
also when the number of partition entries is not a multiple
of sector_size / 128.  Problem diagnosed by Robert Herndon
in http://thread.gmane.org/gmane.comp.gnu.parted.bugs/10173.