x86/cavsdsp: fix buffer alignment in cavs_idct8_add_mmx()

It may be used by ff_add_pixels_clamped_sse2().
Should fix fate-cavs failures on some systems.
Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/x86/cavsdsp.c

@@ -139,7 +139,7 @@ static inline void cavs_idct8_1d(int16_t *block, uint64_t bias)
 static void cavs_idct8_add_mmx(uint8_t *dst, int16_t *block, int stride)
 {
     int i;
-    DECLARE_ALIGNED(8, int16_t, b2)[64];
+    DECLARE_ALIGNED(16, int16_t, b2)[64];
 
     for(i=0; i<2; i++){
         DECLARE_ALIGNED(8, uint64_t, tmp);