Update dtc to fix compilation problem on Mac OS 10.6

Currently QEMU does not build on Mac OS 10.6
because of a missing patch in the dtc
subproject. Updating dtc to make the patch
available fixes this problem.

Signed-off-by: John Arbuckle <programmingkidx@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>