Fix warning in gfx: "variable does not need to be mutable"

Fix this warning:
```
compile: src/servo/build/x86_64-apple-darwin/src/components/gfx/libgfx.dummy
src/servo/src/components/gfx/render_task.rs:304:21: 304:48 warning: variable does not need to be mutable, #[warn(unused_mut)] on by default
src/servo/src/components/gfx/render_task.rs:304                 let mut native_graphics_context = native_graphics_context;
                                                                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~
```