Makefile: Suppress unused-function warning

This commit is to supress warnings caused by the following functions:

* list_del_init
* list_is_last

Author: shengwen-tw

Makefile

@@ -1,7 +1,7 @@
 include mk/common.mk
 
 CC ?= gcc
-CFLAGS := -O2 -g -Wall -Wextra
+CFLAGS := -O2 -g -Wall -Wextra -Wno-unused-function
 CFLAGS += -include common.h
 LDFLAGS :=