Fix compilation when Theora decoder is disabled, but VP3 is enabled.

Originally committed as revision 8682 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/vp3.c

@@ -2646,6 +2646,7 @@ AVCodec vp3_decoder = {
     NULL
 };
 
+#ifdef CONFIG_THEORA_DECODER
 AVCodec theora_decoder = {
     "theora",
     CODEC_TYPE_VIDEO,
@@ -2658,3 +2659,4 @@ AVCodec theora_decoder = {
     0,
     NULL
 };
+#endif