FreeBSD: fix doc build

+ cargo xdoc --target i686-unknown-freebsd --no-default-features --features extra_traits
error[E0412]: cannot find type `ucontext_t` in this scope
   --> src/unix/bsd/freebsdlike/freebsd/mod.rs:166:16
    |
166 |     pub struct ucontext_t {
    |                ^^^^^^^^^^ not found in this scope

Author: valpackett

src/unix/bsd/freebsdlike/freebsd/mod.rs

@@ -160,9 +160,11 @@ s_no_extra_traits! {
         __unused1: ::c_int,
         __unused2: [::c_long; 7]
     }
+}
 
+#[cfg(any(target_arch = "x86_64", target_arch = "aarch64"))]
+s_no_extra_traits! {
     #[cfg_attr(feature = "extra_traits", derive(Debug))]
-    #[cfg(any(target_arch = "x86_64", target_arch = "aarch64"))]
     pub struct ucontext_t {
         pub uc_sigmask: ::sigset_t,
         pub uc_mcontext: ::mcontext_t,