rootfs_remaster: fix fatal missing whitespace

Makefile

@@ -178,7 +178,7 @@ $(call gentargets,$(STATE_DIR)/rootfs_prepared) : $(call archdir,%)$(STATE_DIR)/
 rootfs_remaster : $(ARCH_DIR)$(STATE_DIR)/rootfs_remastered
 $(call gentargets,$(STATE_DIR)/rootfs_remastered) : $(call archdir,%)$(STATE_DIR)/rootfs_extracted | $(APT_CACHE_DIR)
 	$(MAKE) ARCH=$* rootfs_prepare
-ifneq($(strip $(APT_SOURCE_URL_OVERRIDE)),)
+ifneq ($(strip $(APT_SOURCE_URL_OVERRIDE)),)
 	#override apt sources list
 	echo "deb $(APT_SOURCE_URL_OVERRIDE) $(ISO_RELEASE) main restricted universe multiverse" \
 		> "$(call archdir,$*)$(ROOTFS)/etc/apt/sources.list"