Add stub for avx.

src/opts/SkOpts_avx.cpp

Index: src/opts/SkOpts_avx.cpp
diff --git a/src/core/SkSwizzle.cpp b/src/opts/SkOpts_avx.cpp
similarity index 56%
copy from src/core/SkSwizzle.cpp
copy to src/opts/SkOpts_avx.cpp
index f080b83a87870e5f2a528345b9585c90c74c955f..835e0c0ded9e505951187d379f1cf50bd51eaea4 100644
--- a/src/core/SkSwizzle.cpp
+++ b/src/opts/SkOpts_avx.cpp
@@ -5,10 +5,11 @@
  * found in the LICENSE file.
  */
 
-#include "SkSwizzle.h"
-
 #include "SkOpts.h"
 
-void SkSwapRB(uint32_t* dest, const uint32_t* src, int count) {
-    SkOpts::RGBA_to_BGRA(dest, src, count);
+#define SK_OPTS_NS sk_avx
+#include "SkBlend_opts.h"
+
+namespace SkOpts {
+    void Init_avx() { }
 }