swscale: reintroduce sws_format_name() symbol

Reintroduce the internal symbol which was removed in:

commit e1197b9e
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.