datarate_test.cc: remove unused param warnings

Change-Id: I627dc800f1925cc5a5d68630009cf8a454f07567

test/datarate_test.cc

diff --git a/test/datarate_test.cc b/test/datarate_test.cc
index 8dcf26c..a8d8d42 100644
--- a/test/datarate_test.cc
+++ b/test/datarate_test.cc
@@ -41,7 +41,7 @@
   }
 
   virtual void PreEncodeFrameHook(::libvpx_test::VideoSource *video,
-                                  ::libvpx_test::Encoder *encoder) {
+                                  ::libvpx_test::Encoder* /*encoder*/) {
     const vpx_rational_t tb = video->timebase();
     timebase_ = static_cast<double>(tb.num) / tb.den;
     duration_ = 0;