jfs: Disable gcc9 -Waddress-of-packed-member

jfs: Disable gcc9 -Waddress-of-packed-member

Disable the -Wadress-of-packaed-member diagnostic for the
grub_jfs_getent function since the result is found to be false postive.

The leaf is read into memory as continous chunks in size of 32 bytes and
the pointer to its base is aligned, which also guarentee its member
leaf->namepart is aligned.

[   60s] ../grub-core/fs/jfs.c: In function 'grub_jfs_getent':
[   60s] ../grub-core/fs/jfs.c:557:44: error: taking address of packed member of 'struct grub_jfs_leaf_dirent' may result in an unaligned pointer value [-Werror=address-of-packed-member]
[   60s]   557 |   le_to_cpu16_copy (filename + strpos, leaf->namepart, len < diro->data->namecomponentlen ? len
[   60s]       |                                        ~~~~^~~~~~~~~~
[   60s] ../grub-core/fs/jfs.c:570:48: error: taking address of packed member of 'struct grub_jfs_leaf_next_dirent' may result in an unaligned pointer value [-Werror=address-of-packed-member]
[   60s]   570 |  le_to_cpu16_copy (filename + strpos, next_leaf->namepart, len < 15 ? len : 15);
[   60s]       |                                       ~~~~~~~~~^~~~~~~~~~
[   60s] cc1: all warnings being treated as errors

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>