Add -Wunused to keystore

Some build targets use clang which has this on with -Wall and -Wextra so
to be consistent with warnings enable it for all targets.

Change-Id: I63c46d9c516f6c2ce27a7c33c5677b0ee3c22eca
diff --git a/keystore/Android.mk b/keystore/Android.mk
index dc161ce..aa40fd1 100644
--- a/keystore/Android.mk
+++ b/keystore/Android.mk
@@ -20,7 +20,7 @@
 ifeq ($(USE_32_BIT_KEYSTORE), true)
 LOCAL_MULTILIB := 32
 endif
-LOCAL_CFLAGS := -Wall -Wextra -Werror
+LOCAL_CFLAGS := -Wall -Wextra -Werror -Wunused
 LOCAL_SRC_FILES := keystore.cpp keyblob_utils.cpp
 LOCAL_SHARED_LIBRARIES := \
 	libbinder \