Suppress integer overflow warning

Change-Id: Ia6bce316ae27175a15c2fe49f10ed34706069a7f
diff --git a/aom_dsp/variance.c b/aom_dsp/variance.c
index 5050055..05db2be 100644
--- a/aom_dsp/variance.c
+++ b/aom_dsp/variance.c
@@ -446,7 +446,7 @@
     for (j = 0; j < w; ++j) {
       const int diff = a[j] - b[j];
       *sum += diff;
-      *sse += diff * diff;
+      *sse += (int64_t)diff * (int64_t)diff;
     }
     a += a_stride;
     b += b_stride;