avformat/internal: Move ff_read_line_to_bprint_overwrite to avio_internal.h

It only uses an AVIOContext and an AVBPrint.

When doing so, it turned out that several non-users of
ff_read_line_to_bprint_overwrite() and ff_bprint_to_codecpar_extradata()
relied on libavformat/internal.h to include bprint.h or avstring.h
for them. In order to avoid a repeat of this and in order to reduce
unnecessary dependencies, a forward declaration of struct AVBPrint is
used instead of including bprint.h.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/au.c

@@ -27,6 +27,7 @@
  * http://www.goice.co.jp/member/mo/formats/au.html
  */
 
+#include "libavutil/bprint.h"
 #include "avformat.h"
 #include "internal.h"
 #include "avio_internal.h"