l4re: Move pthread_attr_t into s!

Resolve the `improper_ctypes` warning on l4re-uclibc that we are getting
in CI. The current version was added in e412497 ("Move
L4Re-specific code into separate module.") and doesn't seem to have been
intentional.

(backport <#4584>)
(cherry picked from commit 95c1400)

Author: tgross35

src/unix/linux_like/linux/uclibc/x86_64/l4re.rs

@@ -26,22 +26,21 @@ s! {
         /// Bitmap of CPUs.
         map: l4_umword_t,
     }
-}
 
-#[cfg_attr(feature = "extra_traits", derive(Debug))]
-pub struct pthread_attr_t {
-    pub __detachstate: c_int,
-    pub __schedpolicy: c_int,
-    pub __schedparam: super::__sched_param,
-    pub __inheritsched: c_int,
-    pub __scope: c_int,
-    pub __guardsize: size_t,
-    pub __stackaddr_set: c_int,
-    pub __stackaddr: *mut c_void, // better don't use it
-    pub __stacksize: size_t,
-    // L4Re specifics
-    pub affinity: l4_sched_cpu_set_t,
-    pub create_flags: c_uint,
+    pub struct pthread_attr_t {
+        pub __detachstate: c_int,
+        pub __schedpolicy: c_int,
+        pub __schedparam: super::__sched_param,
+        pub __inheritsched: c_int,
+        pub __scope: c_int,
+        pub __guardsize: size_t,
+        pub __stackaddr_set: c_int,
+        pub __stackaddr: *mut c_void, // better don't use it
+        pub __stacksize: size_t,
+        // L4Re specifics
+        pub affinity: l4_sched_cpu_set_t,
+        pub create_flags: c_uint,
+    }
 }
 
 // L4Re requires a min stack size of 64k; that isn't defined in uClibc, but