image-commands.mk: Be consistent in command invocation

Most/all other tools use the staging dir prefix, gzip should as well.
Signed-off-by: NOlliver Schinagl <oliver@schinagl.nl>
Acked-by: NChristian Marangi <ansuelsmth@gmail.com>

include/image-commands.mk

@@ -307,7 +307,7 @@ define Build/fit
 endef
 
 define Build/gzip
-	gzip -f -9n -c $@ $(1) > $@.new
+	$(STAGING_DIR_HOST)/bin/gzip -f -9n -c $@ $(1) > $@.new
 	@mv $@.new $@
 endef