kbuild: use in-kernel unifdef

Let headers_install use in-kernel unifdef

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/scripts/Makefile b/scripts/Makefile
index 6f6b48f..d531a1f 100644
--- a/scripts/Makefile
+++ b/scripts/Makefile
@@ -15,6 +15,9 @@
 
 always		:= $(hostprogs-y)
 
+# The following hostprogs-y programs are only build on demand
+hostprogs-y += unifdef
+
 subdir-$(CONFIG_MODVERSIONS) += genksyms
 subdir-$(CONFIG_MODULES)     += mod