dtc: Add missing copyright notice for dumptrees.c

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> When I released libfdt, I forgot to add a copyright notice to
> dumptrees.c (probably because the program is so trivial).  Apparently
> the lack causes trouble for Debian, so this patch adds one.  I've gone
> through the git history and double checked that no-one has touched
> this file except me (and I barely have myself since its initial
> commit).
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl