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: NDavid Gibson <david@gibson.dropbear.id.au>

dtc @ 558cd81b

-Subproject commit fa8bc7f928ac25f23532afc8beb2073efc8fb063
+Subproject commit 558cd81bdd432769b59bff01240c44f82cfb1a9d