This is already typedef'ed like that in gstffmpeg.h, included from this file.
https://bugs.gentoo.org/show_bug.cgi?id=460208

Index: gst-ffmpeg-0.10.13_p201211/ext/ffmpeg/gstffmpegpipe.h
===================================================================
--- gst-ffmpeg-0.10.13_p201211.orig/ext/ffmpeg/gstffmpegpipe.h
+++ gst-ffmpeg-0.10.13_p201211/ext/ffmpeg/gstffmpegpipe.h
@@ -48,8 +48,6 @@ G_BEGIN_DECLS
   g_cond_signal (m->cond);                                              \
 } G_STMT_END
 
-typedef struct _GstFFMpegPipe GstFFMpegPipe;
-
 struct _GstFFMpegPipe
 {
   /* lock for syncing */