xfs: Fix build breakage in xfs_iops.c when CONFIG_FS_POSIX_ACL is not set

commit 4e34e719, that takes the ACL checks to common code,
accidentely broke the build when CONFIG_FS_POSIX_ACL is not set:

  CC      fs/xfs/linux-2.6/xfs_iops.o
fs/xfs/linux-2.6/xfs_iops.c:1025:14: error: ‘xfs_get_acl’ undeclared here (not in a function)

Fix this by declaring xfs_get_acl a static inline function.

Signed-off-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Signed-off-by: Alex Elder <aelder@sgi.com>