include/kernel-build.mk: fix kernel rebuild on backport patch changes

An incorrect variable name was referenced in KERNEL_FILE_DEPENDS, leading
to the omission of the backport-* patch dirs in the generation of the
prepared stamp name.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

include/kernel-build.mk

@@ -11,7 +11,7 @@ ifneq ($(DUMP),1)
   all: compile
 endif
 
-KERNEL_FILE_DEPENDS=$(BACKPORT_PATCH_DIR) $(GENERIC_PATCH_DIR) $(GENERIC_HACK_DIR) $(PATCH_DIR) $(GENERIC_FILES_DIR) $(FILES_DIR)
+KERNEL_FILE_DEPENDS=$(GENERIC_BACKPORT_DIR) $(GENERIC_PATCH_DIR) $(GENERIC_HACK_DIR) $(PATCH_DIR) $(GENERIC_FILES_DIR) $(FILES_DIR)
 STAMP_PREPARED=$(LINUX_DIR)/.prepared$(if $(QUILT)$(DUMP),,_$(shell $(call find_md5,$(KERNEL_FILE_DEPENDS),)))
 STAMP_CONFIGURED:=$(LINUX_DIR)/.configured
 include $(INCLUDE_DIR)/download.mk