avformat: Identify anonymous AVIO typedef structs.

Anonymous typedef structs prevent forward declaration, this
change gives the AVIOContext and AVIOInterruptCB structures
a name.  These structures are now in line with other common
structures such as AVFormatContext and AVCodecContext.

Signed-off-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Diego Biurrun <diego@biurrun.de>
