x86 store: input is const

Change-Id: Ie0108d79afcc9e29f9d1395d95184daf2e0f9ccb
diff --git a/aom_dsp/x86/synonyms.h b/aom_dsp/x86/synonyms.h
index c6f18ae..2e99bee 100644
--- a/aom_dsp/x86/synonyms.h
+++ b/aom_dsp/x86/synonyms.h
@@ -47,7 +47,7 @@
 }
 
 static INLINE void xx_storel_32(void *const a, const __m128i v) {
-  int val = _mm_cvtsi128_si32(v);
+  const int val = _mm_cvtsi128_si32(v);
   memcpy(a, &val, sizeof(val));
 }