Move TRANSPOSE8 macro to dsputil_altivec.h.

(was duplicated in libavcodec/ppc/vc1dsp_altivec.c
nd libavcodec/ppc/mpegvideo_altivec.c, and will be
used in upcoming routines for h264_altivec.c)

Originally committed as revision 6621 to svn://svn.ffmpeg.org/ffmpeg/trunk