dsp.h: add a check for AVX2 / define WEBP_USE_AVX2

Change-Id: I90cc870f0bb4426af701779c367587dc2ae79c8b
diff --git a/src/dsp/dsp.h b/src/dsp/dsp.h
index ca23562..c046417 100644
--- a/src/dsp/dsp.h
+++ b/src/dsp/dsp.h
@@ -40,6 +40,10 @@
 #define WEBP_USE_SSE2
 #endif
 
+#if defined(__AVX2__)
+#define WEBP_USE_AVX2
+#endif
+
 #if defined(__ANDROID__) && defined(__ARM_ARCH_7A__)
 #define WEBP_ANDROID_NEON  // Android targets that might support NEON
 #endif