Change the type of Y table to pointer to void in fill_table().

Change the type of Y table to pointer to void in fill_table().
This fixes warnings about wrong type being used, e.g.:
libswscale/yuv2rgb.c: In function ‘ff_yuv2rgb_c_init_tables’:
libswscale/yuv2rgb.c:778: warning: passing argument 4 of ‘fill_table’ from incompatible pointer type
libswscale/yuv2rgb.c:598: note: expected ‘uint8_t *’ but argument is of type ‘uint16_t *’

Originally committed as revision 31722 to svn://svn.mplayerhq.hu/mplayer/trunk/libswscale