Suppress the compiler warning about raw_pointer_deriving in layout/context.rs.

This suppress the following compile warning:
```
compile: servo
src/components/main/layout/context.rs:75:18: 75:26 warning: use of `#[deriving]` with a raw pointer, #[warn(raw_pointer_deriving)] on by default
src/components/main/layout/context.rs:75     pub stylist: *Stylist,
                                                          ^~~~~~~~
```
Merge pull request #2714 from saneyuki/fix_warn

Suppress the compiler warning about raw_pointer_deriving in layout/context.rs