xfsprogs: don't warn about packed members

xfsprogs: don't warn about packed members

gcc 9.2.1 throws lots of new warnings during the build like this:

xfs_format.h:790:3: warning: taking address of packed member of ‘struct xfs_agfl’ may result in an unaligned pointer value [-Waddress-of-packed-member]
  790 |   &(XFS_BUF_TO_AGFL(bp)->agfl_bno[0]) : \
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
xfs_alloc.c:3149:13: note: in expansion of macro ‘XFS_BUF_TO_AGFL_BNO’
 3149 |  agfl_bno = XFS_BUF_TO_AGFL_BNO(mp, agflbp);
      |             ^~~~~~~~~~~~~~~~~~~

We know this packed structure aligned correctly, so turn off this
warning to shut gcc up.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
[sandeen: kernel has done this globally as well in 6f303d60534]
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>