postproc/yuv2rgb_altivec.c compile fix

yuv2rgb_altivec_init_tables does initialize the SwsContext vectors.
missing vec_splat.
patch by (Luca Barbato <lu_zero at gentoo dot org>) and (Romain Dolbeau <dolbeau at irisa dot fr>)

Originally committed as revision 13565 to svn://svn.mplayerhq.hu/mplayer/trunk/postproc