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


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@6195 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavformat/utils.c

@@ -474,6 +474,9 @@ static const AVOption options[]={
 
 static const AVClass av_format_context_class = { "AVFormatContext", format_to_name, options };
 
+#if LIBAVCODEC_VERSION_INT > ((52<<16)+(0<<8)+0)
+static
+#endif
 void avformat_get_context_defaults(AVFormatContext *s){
     memset(s, 0, sizeof(AVFormatContext));