kbuild: Fix make O=

A bug that slipped through when introducing Makefile.host.
A good way to check if people actually uses make O= ;-)
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/Makefile.host

@@ -73,6 +73,7 @@ host-cxxmulti	:= $(addprefix $(obj)/,$(host-cxxmulti))
 host-cxxobjs	:= $(addprefix $(obj)/,$(host-cxxobjs))
 host-cshlib	:= $(addprefix $(obj)/,$(host-cshlib))
 host-cshobjs	:= $(addprefix $(obj)/,$(host-cshobjs))
+obj-dirs        := $(addprefix $(obj)/,$(obj-dirs))
 
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 # Handle options to gcc. Support building with separate output directory