gpt_partition: Mark grub_gpt_partentry as having natural alignment

gpt_partition: Mark grub_gpt_partentry as having natural alignment

gpt_partition contains grub_guid. We need to decide whether the whole
structure is unaligned and then we need to use packed_guid. But we never
have unaligned part entries as we read them in an aligned buffer from disk.
Hence just make it all aligned.

Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>