Fix linking with H.261 support disabled.

patch by Tom Harper, d.thomas.harper gmail com

Originally committed as revision 7134 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpegvideo.c

@@ -1207,6 +1207,7 @@ int MPV_encode_init(AVCodecContext *avctx)
         avctx->delay=0;
         s->low_delay=1;
         break;
+#ifdef CONFIG_H261_ENCODER
     case CODEC_ID_H261:
         if (ff_h261_get_picture_format(s->width, s->height) < 0) {
             av_log(avctx, AV_LOG_ERROR, "The specified picture size of %dx%d is not valid for the H.261 codec.\nValid sizes are 176x144, 352x288\n", s->width, s->height);
@@ -1216,6 +1217,7 @@ int MPV_encode_init(AVCodecContext *avctx)
         avctx->delay=0;
         s->low_delay=1;
         break;
+#endif
     case CODEC_ID_H263:
         if (h263_get_picture_format(s->width, s->height) == 7) {
             av_log(avctx, AV_LOG_INFO, "The specified picture size of %dx%d is not valid for the H.263 codec.\nValid sizes are 128x96, 176x144, 352x288, 704x576, and 1408x1152. Try H.263+.\n", s->width, s->height);