Make static constants const for Visual C. Allows optimizer to collapse similar…

Make static constants const for Visual C.  Allows optimizer to collapse similar constants (SSE2 is subset of AVX2).  GCC has compiler bug with const attributes, but Visual C does not.
BUG=254
TEST=windows build still passes and is smaller
R=cdelaunay@google.com

Review URL: https://webrtc-codereview.appspot.com/2200007

git-svn-id: http://libyuv.googlecode.com/svn/trunk@788 16f28f9a-4ce2-e073-06de-1de4eb20be90

README.chromium

 Name: libyuv
 URL: http://code.google.com/p/libyuv/
-Version: 787
+Version: 788
 License: BSD
 License File: LICENSE
 

include/libyuv/version.h

@@ -11,6 +11,6 @@
 #ifndef INCLUDE_LIBYUV_VERSION_H_  // NOLINT
 #define INCLUDE_LIBYUV_VERSION_H_
 
-#define LIBYUV_VERSION 787
+#define LIBYUV_VERSION 788
 
 #endif  // INCLUDE_LIBYUV_VERSION_H_  NOLINT