svq3: fix pointer type warning

Fixes:
libavcodec/svq3.c:661:9: warning: passing argument 2 of 'svq3_decode_block' from incompatible pointer type
libavcodec/svq3.c:208:19: note: expected 'DCTELEM *' but argument is of type 'DCTELEM (*)[32]'

Signed-off-by: Mans Rullgard <mans@mansr.com>