Skip mpegaudio3.h in checkheaders

This unused header is a placeholder for work in progress(?).

This makes checkheaders pass again.

Originally committed as revision 22338 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/Makefile

@@ -715,6 +715,7 @@ SKIPHEADERS-$(CONFIG_LIBSCHROEDINGER)  += libschroedinger.h
 SKIPHEADERS-$(CONFIG_VAAPI)            += vaapi_internal.h
 SKIPHEADERS-$(CONFIG_VDPAU)            += vdpau.h
 SKIPHEADERS-$(CONFIG_XVMC)             += xvmc.h
+SKIPHEADERS                            += mpegaudio3.h
 
 EXAMPLES = api