Reintroduce the internal symbol which was removed in:
commit e1197b9e1746c03b1d13d816d1569aeaf1b71ecc
Author: Stefano Sabatini <stefano.sabatini-lala@poste.it>
Date: Sun May 29 17:57:40 2011 +0200
swscale: remove sws_format_name()
Use av_get_pix_fmt_name() instead.
The symbol is used by some external libs (hi libx264!), this gives
time to them to use the recently added av_get_pix_fmt_name() rather
than an internal symbol.