image-commands.mk: Be consistent in command invocation

Most/all other tools use the staging dir prefix, gzip should as well.

Signed-off-by: Olliver Schinagl <[email protected]>
Acked-by: Christian Marangi <[email protected]>

include/image-commands.mk

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