Turn on -Wunused and fix the mistakes it uncovers.

Change-Id: I023d2d8b547fbc21d4124bb7510d42b06a0dc501
diff --git a/benchmarks/Android.mk b/benchmarks/Android.mk
index cf3d692..f9722ae 100644
--- a/benchmarks/Android.mk
+++ b/benchmarks/Android.mk
@@ -24,7 +24,7 @@
 
 benchmark_c_flags = \
     -O2 \
-    -Wall -Wextra \
+    -Wall -Wextra -Wunused \
     -Werror \
     -fno-builtin \
     -std=gnu++11 \