kbuild: add dtc as dependency on .dtb files

[Linux commit b8fc5b2157b1]

If dtc is rebuilt, we should rebuild .dtb files with the new dtc.

[Import notes: Back then there was no .dtbo rule in Linux's
Makefile.lib, but the current .dtbo rules in Linux also have the
$(DTC) dependency, so also add it to our .dtbo rule.]

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>