Fix SDL2 build regression caused by undefined Key usage

3e075586ead63816d93c5df5871a4a11bbd85edd ("To prevent undesirable release of
analogue joystick while using touchpad.") used SDLK_LSUPER from SDL1.2 which is
not used anymore in mupen64plus-input-sdl. Instead SDL2 keys must be used and
only the SDL1.2 compatibility wrapper can reference SDLK_*
Merge pull request #8 from fayvel/sdl_key

Fix SDL2 build regression caused by undefined Key usage