py: Add order-only dependency for py-version.h

Else the directory might not exist.

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
diff --git a/py/py.mk b/py/py.mk
index a821b7c..0975169 100644
--- a/py/py.mk
+++ b/py/py.mk
@@ -123,7 +123,7 @@
 FORCE:
 .PHONY: FORCE
 
-$(HEADER_BUILD)/py-version.h: FORCE
+$(HEADER_BUILD)/py-version.h: FORCE | $(HEADER_BUILD)
 	$(Q)$(PY_SRC)/py-version.sh > $@.tmp
 	$(Q)if [ -f "$@" ] && cmp -s $@ $@.tmp; then rm $@.tmp; else echo "Generating $@"; mv $@.tmp $@; fi