package/mender/mender.mk: add missing backslash

Without this backslash, any attempt to run make will result in the error:
package/mender/mender.mk:44: *** recipe commences before first target.  Stop

Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/mender/mender.mk

@@ -40,7 +40,7 @@ MENDER_LDFLAGS = -X main.Version=$(MENDER_VERSION)
 MENDER_UPDATE_MODULES_FILES = \
 	directory \
 	script \
-	single-file
+	single-file \
 	$(if $(BR2_PACKAGE_DOCKER_CLI),docker) \
 	$(if $(BR2_PACKAGE_RPM),rpm)