avcodec/mpeg4videodec: Align idct-block appropriately

It is accessed via AV_RN64A in ff_simple_idct_put_int32_10bit().
Should fix the UBSan failures in the mpeg4-simple-studio-profile
test here:
https://fate.ffmpeg.org/report.cgi?time=20240312011016&slot=ppc-linux-gcc-13.2-ubsan-altivec-qemu

Reviewed-by: Kieran Kunhya <kierank@obe.tv>
Tested-by: Sean McGovern <gseanmcg@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/mpeg4videodec.h

@@ -29,6 +29,7 @@
 #include "mpegvideo.h"
 #include "mpeg4videodsp.h"
 
+#include "libavutil/mem_internal.h"
 
 typedef struct Mpeg4DecContext {
     MpegEncContext m;
@@ -83,7 +84,7 @@ typedef struct Mpeg4DecContext {
 
     Mpeg4VideoDSPContext mdsp;
 
-    int32_t block32[12][64];
+    DECLARE_ALIGNED(8, int32_t, block32)[12][64];
     // 0 = DCT, 1 = DPCM top to bottom scan, -1 = DPCM bottom to top scan
     int dpcm_direction;
     int16_t dpcm_macroblock[3][256];