Export the four remaining H.264 decoder intra prediction functions for reuse in

the H.264 encoder. These functions are: pred8x8_left_dc_c, pred8x8_top_dc_c,
pred16x16_left_dc_c and pred16x16_top_dc_c.

Originally committed as revision 9107 to svn://svn.ffmpeg.org/ffmpeg/trunk