Don't generate warning when deriving on deprecated type

[joshlf]

While working on this PR, I wrote the following code:

    #[cfg_attr(feature = "zerocopy", derive(FromZeroes, FromBytes, AsBytes))]    
    #[deprecated(                                                                
        since = "0.2.55",                                                        
        note = "Use the `mach2` crate instead",                                  
    )]                                                                           
    pub struct mach_timebase_info {                                              
        pub numer: u32,                                                          
        pub denom: u32,                                                          
    }

Running cargo check --features zerocopy on cargo 1.74.0-nightly (b4ddf95ad 2023-09-18), I get the following warning:

warning: use of deprecated struct `unix::bsd::apple::mach_timebase_info`: Use the `mach2` crate instead
   --> src/unix/bsd/apple/mod.rs:306:16
    |
306 |     pub struct mach_timebase_info {
    |                ^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(deprecated)]` on by default

Mentoring instructions

• Figure out the conditions under which this warning is emitted
• Write a test that reproduces this behavior
• Modify our derives as needed to not generate this warning
  • Done: [derive] Emit #[allow(deprecated)] in derived code #556