makes avformat_get_context_defaults() static

when the next compatibility breaking change occurs, as suggested by
Michael Niedermayer here:
http://article.gmane.org/gmane.comp.video.ffmpeg.devel/36377
Patch by Takis
Original thread:
Date: Sep 8, 2006 11:19 AM
Subject: [Ffmpeg-devel] [PATCH] Make avformat_get_context_defaults static

Originally committed as revision 6195 to svn://svn.ffmpeg.org/ffmpeg/trunk