[SDL] Workaround for a Mono crash in SDL.JoystickGetGUID

It appears that Mono 3.4.0 fails to marshal SDL_JoystickGUID
correctly when used as a return value. Changing the fixed buffer
to a couple of long fields allows the struct to be marshalled
correctly.
Merge pull request #163 from thefiddler/sdl_guid_crash

[SDL] Workaround for a Mono crash in SDL.JoystickGetGUID