[XFS] Fix up xfs out-of-tree builds. (a.k.a. external modules)

Change -I include directives to find headers in the out-of-tree spot. This
allows a directory containing only xfs files to be built as:

SGI-PV: 971186
SGI-Modid: xfs-linux-melb:xfs-kern:29878a

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Donald Douwsma <donaldd@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>