vp9_avg_intrin_sse2: correct intrinsics include

immintrin.h -> emmintrin.h
fixes build where newer intrinsics are unavailable

Change-Id: I79311b39bfa782fc2abeb45884ecb417050cb9f8

vp9/encoder/x86/vp9_avg_intrin_sse2.c

@@ -8,7 +8,7 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#include <immintrin.h>
+#include <emmintrin.h>
 #include "vpx_ports/mem.h"