add stub dsp/enc_avx2.c

VP8EncDspInitAVX2 is included in sse2 builds for now, later a configure
flag should be added to avoid the stub when avx2 is unavailable/disabled

Change-Id: I6127b687c273f46f41652aaf8e3b86ae3cfb8108

src/dsp/enc_avx2.c

@@ -0,0 +1,24 @@
+// Copyright 2014 Google Inc. All Rights Reserved.
+//
+// Use of this source code is governed by a BSD-style license
+// that can be found in the COPYING file in the root of the source
+// tree. An additional intellectual property rights grant can be found
+// in the file PATENTS. All contributing project authors may
+// be found in the AUTHORS file in the root of the source tree.
+// -----------------------------------------------------------------------------
+//
+// AVX2 version of speed-critical encoding functions.
+
+#include "./dsp.h"
+
+#if defined(WEBP_USE_AVX2)
+
+#endif  // WEBP_USE_AVX2
+
+//------------------------------------------------------------------------------
+// Entry point
+
+extern void VP8EncDspInitAVX2(void);
+
+void VP8EncDspInitAVX2(void) {
+}