media: syntax modifications for native Windows build

- includes <functional> in media_buffer.h for std::function.

- forward declare AudioFrame and DeviceParams as structs in
  media_decoder.h, as they are both defined as such.

- remove AVRational compound literal from media_encoder.cpp.

- use a #pragma message instead of a #warning in Visual Studio

Change-Id: If32cfbb7504d65c9687814dcc3b5dfa859017fc5
Tuleap: #790
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>