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>

diff --git a/dtc b/dtc
index 558cd81..e543880 160000 (submodule)
--- a/dtc
+++ b/dtc
@@ -1 +1 @@
-Subproject commit 558cd81bdd432769b59bff01240c44f82cfb1a9d
+Subproject commit e54388015af1fb4bf04d0bca99caba1074d9cc42