Use -Werror in external/dhcpcd-6.8.2

* Suppress warnings of unused variables.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I981ad76ad8f79a2c1ebd05be2bdc91c98aae8e18
diff --git a/Android.mk b/Android.mk
index 36ab085..9d1d8c5 100644
--- a/Android.mk
+++ b/Android.mk
@@ -44,7 +44,7 @@
     compat/posix_spawn.c
 
 # Always support IPv4.
-LOCAL_CFLAGS += -DINET
+LOCAL_CFLAGS += -DINET -Wall -Werror -Wno-unused-variable
 
 ifeq ($(DHCPCD_USE_SCRIPT), yes)
 LOCAL_SRC_FILES += script.c