Add missing boxarts to Nintendo - GameCube

[mfbruno]

hey guys, i was messing around with Dolphin today and decided to put here two famous boxarts that was missing in Nintendo - GameCube

Metroid Prime (USA) (v1.02)

Metroid Prime (USA)

Viewtiful Joe (USA)

[RobLoach]

Split the repository up into individual systems to make contribution easier:
https://github.com/libretro-thumbnails/libretro-thumbnails

You'll find GameCube over at:
https://github.com/libretro-thumbnails/Nintendo_-_GameCube