xfs: fix tracing builds inside the source tree

The code really requires the current source directory to be in the
header search path.  We already do this if building with an object
tree separate from the source, but it needs to be added manually
if building inside the source.  The cflags addition for it accidentally
got removed when collapsing the xfs directory structure.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Alex Elder <aelder@sgi.com>