kbuild: add dtc as dependency on .dtb files

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

Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Rob Herring <robh@kernel.org>
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 1bc2f90..6aeb915 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -316,7 +316,7 @@
 		-d $(depfile).dtc.tmp $(dtc-tmp) ; \
 	cat $(depfile).pre.tmp $(depfile).dtc.tmp > $(depfile)
 
-$(obj)/%.dtb: $(src)/%.dts FORCE
+$(obj)/%.dtb: $(src)/%.dts $(DTC) FORCE
 	$(call if_changed_dep,dtc)
 
 dtc-tmp = $(subst $(comma),_,$(dot-target).dts.tmp)