[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: NEric Sandeen <sandeen@sandeen.net>
Signed-off-by: NDonald Douwsma <donaldd@sgi.com>
Signed-off-by: NLachlan McIlroy <lachlan@sgi.com>

fs/xfs/Makefile

@@ -16,7 +16,7 @@
 # Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 #
 
-EXTRA_CFLAGS +=	 -Ifs/xfs -Ifs/xfs/linux-2.6 -funsigned-char
+EXTRA_CFLAGS +=	 -I$(src) -I$(src)/linux-2.6 -funsigned-char
 
 XFS_LINUX := linux-2.6