Remove unused imports

Just silencing some warnings:

```
robn@pyro:~/code/rust/rust-sdl$ make
rustc --cfg image --cfg mixer src/sdl.rc -o libsdl.dummy
src/start.rs:5:4: 5:27 warning: unused import
src/start.rs:5 use core::task::local_data;
                   ^~~~~~~~~~~~~~~~~~~~~~~
src/start.rs:3:17: 3:24 warning: unused import
src/start.rs:3 use core::libc::{c_char, c_int};
                                ^~~~~~~
src/start.rs:1:4: 1:26 warning: unused import
src/start.rs:1 use core::cast::transmute;
                   ^~~~~~~~~~~~~~~~~~~~~~
src/start.rs:3:25: 3:31 warning: unused import
src/start.rs:3 use core::libc::{c_char, c_int};
                                        ^~~~~~
src/mixer.rs:6:24: 6:31 warning: unused import
src/mixer.rs:6 use core::libc::{c_int, malloc, size_t};
                                       ^~~~~~~
src/mixer.rs:6:32: 6:39 warning: unused import
src/mixer.rs:6 use core::libc::{c_int, malloc, size_t};
                                               ^~~~~~~
touch libsdl.dummy
```