set global header flag for all media types

[elkq]

When the output format requires global headers,
CODEC_FLAG_GLOBAL_HEADER should be set for all codec media types.

This appears to be a very old bug, inherited from the 2009 version of output_example.c

This fix uses the same approach currently in new_output_stream()
in both avconv_opt.c and ffmpeg_opt.c (just search for CODEC_FLAG_GLOBAL_HEADER)

It is also present in a slightly different form in add_audio_stream() of libavformat/output-example.c
see: libav change 073189917e4dfa0640e8180110b07e130471f37f
Make output-example.c handle AAC audio.
Patch by Martin Storsjö martin martin st
Originally committed as revision 21367 to svn://svn.ffmpeg.org/ffmpeg/trunk

This change may fix the residual issues from "Leading silence (gaps) in audio segments",
but that needs to be confirmed.