Fix packed-not-aligned error on GCC 8

When building with GCC 8, there are several errors regarding packed-not-aligned.

./include/grub/gpt_partition.h:79:1: error: alignment 1 of ‘struct grub_gpt_partentry’ is less than 8 [-Werror=packed-not-aligned]

This patch fixes the build error by cleaning up the ambiguity of placing
aligned structure in a packed one. In "struct grub_btrfs_time" and "struct
grub_gpt_part_type", the aligned attribute seems to be superfluous, and also
has to be packed, to ensure the structure is bit-to-bit mapped to the format
laid on disk. I think we could blame to copy and paste error here for the
mistake. In "struct efi_variable", we have to use grub_efi_packed_guid_t, as
the name suggests. :)

Signed-off-by: Michael Chang <mchang@suse.com>
Tested-by: Michael Chang <mchang@suse.com>
Tested-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/fs/btrfs.c

@@ -175,7 +175,7 @@ struct grub_btrfs_time
 {
   grub_int64_t sec;
   grub_uint32_t nanosec;
-} __attribute__ ((aligned (4)));
+} GRUB_PACKED;
 
 struct grub_btrfs_inode
 {