ART: Add -Wunused

Until the global CFLAGS are fixed, add Wunused. Fix declarations
in the optimizing compiler.

Change-Id: Ic4553f08e809dc54f3d82af57ac592622c98e000
diff --git a/build/Android.common_build.mk b/build/Android.common_build.mk
index 3000cdf..36c9342 100644
--- a/build/Android.common_build.mk
+++ b/build/Android.common_build.mk
@@ -191,6 +191,7 @@
   -Wunreachable-code \
   -Wredundant-decls \
   -Wshadow \
+  -Wunused \
   -fvisibility=protected \
   $(art_default_gc_type_cflags)