package/swupdate: add missing backslash

Commit 0b4a9930124f7fb4c7bfae281db900905006a2da ("package/swupdate:
specify SWU_VER during build") forgot a backslash.

Fixes the following check-package warning:

package/swupdate/swupdate.mk:213: unexpected indent with tabs

and obviously makes the code correct.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/swupdate/swupdate.mk

@@ -209,7 +209,7 @@ SWUPDATE_KCONFIG_FILE = $(call qstrip,$(BR2_PACKAGE_SWUPDATE_CONFIG))
 SWUPDATE_KCONFIG_EDITORS = menuconfig xconfig gconfig nconfig
 
 SWUPDATE_MAKE_OPTS = \
-	SWU_VER="$(SWUPDATE_VERSION) (Buildroot $(BR2_VERSION_FULL))"
+	SWU_VER="$(SWUPDATE_VERSION) (Buildroot $(BR2_VERSION_FULL))" \
 	CROSS_COMPILE="$(TARGET_CROSS)" \
 	CONFIG_EXTRA_CFLAGS="$(TARGET_CFLAGS)" \
 	CONFIG_EXTRA_LDFLAGS="$(TARGET_LDFLAGS)"