dtc: Update submodule to avoid build errors

The currently included version of the dtc/libfdt submodule has some build
errors on certain distributions (including RHEL7).  This is due to some
poorly named macros in libfdt.h; they're designed for use with the sparse
static checker, but use reserved names which conflict with some symbols in
the standard headers.

That's been corrected in upstream dtc, this updates the qemu submodule to
bring the fix to qemu.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
