ich_descriptors_tool/Makefile: Add missing `override` for CFLAGS

This allows to build `ich_descriptors_tool` with custom CFLAGS in the
environment.

Change-Id: If10f08e44141d05f0ee52615303652da576d2fc6
Signed-off-by: Nico Huber <nico.h@gmx.de>
Original-Reviewed-on: https://review.coreboot.org/c/flashrom/+/66979
Original-Reviewed-by: Thomas Heijligen <src@posteo.de>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/72350
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/util/ich_descriptors_tool/Makefile b/util/ich_descriptors_tool/Makefile
index 0b8a210..0891599 100644
--- a/util/ich_descriptors_tool/Makefile
+++ b/util/ich_descriptors_tool/Makefile
@@ -21,7 +21,7 @@
 # If the user has specified custom CFLAGS, all CFLAGS settings below will be
 # completely ignored by gnumake.
 CFLAGS ?= -Os -Wall -Wshadow
-CFLAGS += -I$(SHAREDSRCDIR)/include
+override CFLAGS += -I$(SHAREDSRCDIR)/include
 
 # Auto determine HOST_OS and TARGET_OS if they are not set as argument
 HOST_OS ?= $(shell uname)