kbuild: use in-kernel unifdef

Let headers_install use in-kernel unifdef

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/Makefile b/Makefile
index edfc2fd..b06b1ea 100644
--- a/Makefile
+++ b/Makefile
@@ -893,8 +893,8 @@
 export INSTALL_HDR_PATH
 
 PHONY += headers_install
-headers_install: include/linux/version.h
-	$(Q)unifdef -Ux /dev/null
+headers_install: include/linux/version.h scripts_basic FORCE
+	$(Q)$(MAKE) $(build)=scripts scripts/unifdef
 	$(Q)rm -rf $(INSTALL_HDR_PATH)/include
 	$(Q)$(MAKE) -rR -f $(srctree)/scripts/Makefile.headersinst obj=include