Only exclude VP8 when it is included

If configured with --disable-vp8 kVP8TestVectors is undefined.

Change-Id: I9d83e805923977ab057ab48042580ce0d817ba68

test/external_frame_buffer_test.cc

@@ -232,8 +232,10 @@ TEST_P(ExternalFrameBufferMD5Test, ExtFBMD5Match) {
   const int num_buffers = 13;
   set_num_buffers(num_buffers);
 
+#if CONFIG_VP8_DECODER
   // Tell compiler we are not using kVP8TestVectors.
   (void)libvpx_test::kVP8TestVectors;
+#endif
 
   // Open compressed video file.
   if (filename.substr(filename.length() - 3, 3) == "ivf") {