invalid_file_test: correct HandlePeekResult signature

restore const lost in:
f0f9ab68 invalid_file_test.cc: remove unused param warnings

Change-Id: I0c5318f84c81e3dd7bb44468463faf9edd6bc18a

test/invalid_file_test.cc

@@ -126,7 +126,7 @@ VP9_INSTANTIATE_TEST_CASE(InvalidFileTest,
 class InvalidFileInvalidPeekTest : public InvalidFileTest {
  protected:
   InvalidFileInvalidPeekTest() : InvalidFileTest() {}
-  virtual void HandlePeekResult(libvpx_test::Decoder* /*decoder*/,
+  virtual void HandlePeekResult(libvpx_test::Decoder *const /*decoder*/,
                                 libvpx_test::CompressedVideoSource* /*video*/,
                                 const vpx_codec_err_t /*res_peek*/) {}
 };