Do not declare *_sys_t typedefs globally in vlc_common.h. Instead,
declare them locally in each module that provides a definition.

This paves the way to move C++ definitions into anonymous namespaces in
order to respect C++ ODR.

The picture_resource_t and sout_stream_id_sys_t typedefs will be handled
separately, since they require specific additional changes.

See #18033
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>