Suppress 'extern crate' warning for rust 2018

[jrobsonchase]

With the 2018 preview, and presumably when the edition is officially released, extern crate will be unidiomatic, and will generate warnings.

This suppresses the warning for the extern crate generated in the Serialize and Deserialize derives.

[jrobsonchase]

So this might not actually be needed in the long run. From IRC:

 @steveklabnik | jechase: generally you don't do that; you pick one edition and write your code in it. editions are compatible, so if you write your  
               | code in rust 2018, programs using rust 2015 can use it, and vice versa                                                               
       jechase | Right, but this isn't me writing code, this is serde_derive writing code. I guess the alternative is "don't generate warnings inside 
               | of macros?"                                                                                                                          
 @steveklabnik | ah so                                                                                                                                
 @steveklabnik | i *think* the intention is that macros carry the edition they were deifned in                                                        
 @steveklabnik | so that this doesn't just happen                                                                                                     
 @steveklabnik | i *think* 

[dtolnay]

Thanks! If the compiler ever eliminates this warning then we can remove these lines.