swscale: reintroduce sws_format_name() symbol

Reintroduce the internal symbol which was removed in:

commit e1197b9e17
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.

libswscale/utils.c

@@ -165,6 +165,13 @@ int sws_isSupportedOutput(enum PixelFormat pix_fmt)
 
 extern const int32_t ff_yuv2rgb_coeffs[8][4];
 
+#if FF_API_SWS_FORMAT_NAME
+const char *sws_format_name(enum PixelFormat format)
+{
+    return av_get_pix_fmt_name(format);
+}
+#endif
+
 static double getSplineCoeff(double a, double b, double c, double d, double dist)
 {
     if (dist<=1.0) return ((d*dist + c)*dist + b)*dist +a;