Fix another unused parameter.

Don't know why I don't see this locally; it's genuine (found by the
build server).

Change-Id: I9bbc3e1c8a08ffa50b1a36503b2fa73b26fd3f02
diff --git a/src/dex_instruction_visitor_test.cc b/src/dex_instruction_visitor_test.cc
index 6dbc4d2..8f42b0c 100644
--- a/src/dex_instruction_visitor_test.cc
+++ b/src/dex_instruction_visitor_test.cc
@@ -35,7 +35,7 @@
 
   CountVisitor() : count_(0) {}
 
-  void Do_Default(const Instruction* inst) {
+  void Do_Default(const Instruction*) {
     ++count_;
   }
 };