sdl: audio: fix compile error when running on SDL2 <2.0.4
sdl: audio: fix compile error when running on SDL2 <2.0.4

Signed-off-by: Lilis Iskandar <f9acb790f2c75b127492316321ece0121b8eed9b@veandco.org>

veandco/go-sdl2,veandco/go-sdl2