vp78: Align the intra4x4_pred_mode_top array within VP8Macroblock

This array is written using AV_WN32A, assuming alignment.

This hopefully fixes the failing vp7 fate test on sparc.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/vp8.h

@@ -91,7 +91,7 @@ typedef struct VP8Macroblock {
     uint8_t chroma_pred_mode;
     uint8_t segment;
     uint8_t intra4x4_pred_mode_mb[16];
-    uint8_t intra4x4_pred_mode_top[4];
+    DECLARE_ALIGNED(4, uint8_t, intra4x4_pred_mode_top)[4];
     VP56mv mv;
     VP56mv bmv[16];
 } VP8Macroblock;