barebox: unbreak custom patch handling after 0eba4759 (packages: apply custom patches using *.patch instead of <pkg>-*.patch)

The commit accidently dropped the continuation character (\), breaking the logic.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/barebox/barebox.mk

@@ -30,7 +30,7 @@ BAREBOX_LICENSE_FILES = COPYING
 
 ifneq ($(call qstrip,$(BR2_TARGET_BAREBOX_CUSTOM_PATCH_DIR)),)
 define BAREBOX_APPLY_CUSTOM_PATCHES
-	$(APPLY_PATCHES) $(@D)
+	$(APPLY_PATCHES) $(@D) \
 		$(BR2_TARGET_BAREBOX_CUSTOM_PATCH_DIR) \*.patch
 endef