SDL2 uses SDLCALL to specify a calling convention. On OS/2, it's defined
to `_System' which is similar to `_cdecl' but does not prepend '_'.

After all, without a header, a function is used without `_System'. And
linker will try to `_func' but fail because the function is `func' not
`_func'.
Reviewed-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>