Fix unused variable issues

Also adds -Wunused to bring gcc's -Werror inline with clang's to prevent
similar build errors later.

Bug:21583577
Change-Id: Ia051adbb3ea92a8ace914ad958a73348d70cca17
diff --git a/android_keymaster_test.cpp b/android_keymaster_test.cpp
index f567c72..0f7beea 100644
--- a/android_keymaster_test.cpp
+++ b/android_keymaster_test.cpp
@@ -33,7 +33,7 @@
 extern "C" {
 int __android_log_print(int prio, const char* tag, const char* fmt);
 int __android_log_print(int prio, const char* tag, const char* fmt) {
-    prio, tag, fmt;
+    (void)prio, (void)tag, (void)fmt;
     return 0;
 }
 }  // extern "C"