include snow encoder only if it is not disabled

Originally committed as revision 9494 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/snow.c

@@ -4535,7 +4535,7 @@ AVCodec snow_decoder = {
     NULL
 };
 
-#ifdef CONFIG_ENCODERS
+#ifdef CONFIG_SNOW_ENCODER
 AVCodec snow_encoder = {
     "snow",
     CODEC_TYPE_VIDEO,