vf_boxblur: cosmetics to reduce diff to qatar

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_boxblur.c

@@ -226,9 +226,9 @@ static inline void blur(uint8_t *dst, int dst_step, const uint8_t *src, int src_
      * and subtracting 1 input pixel.
      * The following code adopts this faster variant.
      */
-    int x, sum = 0;
     const int length = radius*2 + 1;
     const int inv = ((1<<16) + length/2)/length;
+    int x, sum = 0;
 
     for (x = 0; x < radius; x++)
         sum += src[x*src_step]<<1;