decode: Use abs() instead of labs()

Both inputs and the assigned variable here are int, so there's no
need to use labs() which uses long (which varies in size among
supported platforms).

src/decode.c

@@ -331,8 +331,8 @@ static void derive_warpmv(const Dav1dTileContext *const t,
     int mvd[8], ret = 0;
     const int thresh = 4 * iclip(imax(bw4, bh4), 4, 28);
     for (int i = 0; i < np; i++) {
-        mvd[i] = labs(pts[i][1][0] - pts[i][0][0] - mv.x) +
-                 labs(pts[i][1][1] - pts[i][0][1] - mv.y);
+        mvd[i] = abs(pts[i][1][0] - pts[i][0][0] - mv.x) +
+                 abs(pts[i][1][1] - pts[i][0][1] - mv.y);
         if (mvd[i] > thresh)
             mvd[i] = -1;
         else