Makefile: release: make .tar.gz reproducible

Drop the timestamp from the .tar.gz to make it reproducible, similar to how
it is done in support/download/helpers.

Notice: .xz files do not contain a timestamp

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

Makefile

@@ -1255,7 +1255,7 @@ release:
 	$(MAKE) O=$(OUT) distclean
 	tar rf $(OUT).tar --owner=0 --group=0 \
 		--mtime="$$(git log -1 --pretty=format:%ci)" $(OUT)
-	gzip -9 -c < $(OUT).tar > $(OUT).tar.gz
+	gzip -9 -n -c < $(OUT).tar > $(OUT).tar.gz
 	xz -9 -c < $(OUT).tar > $(OUT).tar.xz
 	rm -rf $(OUT) $(OUT).tar