nuttx: add __errno() function declaration

Add missing __errno() function declaration for NuttX platform to enable
proper errno handling in Rust code. NuttX's libc provides __errno()
function which returns a pointer to the thread-local errno variable.

This change allows Rust programs running on NuttX to properly access and
manipulate errno values through the standard libc interface.

Fixes errno functionality for NuttX targets.

(backport <rust-lang#4687>)
(cherry picked from commit ffab35a)

Author: no1wudi

src/unix/nuttx/mod.rs

@@ -566,6 +566,7 @@ pub const IP_ADD_MEMBERSHIP: i32 = 0x14;
 pub const IP_DROP_MEMBERSHIP: i32 = 0x15;
 
 extern "C" {
+    pub fn __errno() -> *mut c_int;
     pub fn bind(sockfd: i32, addr: *const sockaddr, addrlen: socklen_t) -> i32;
     pub fn ioctl(fd: i32, request: i32, ...) -> i32;
     pub fn dirfd(dirp: *mut DIR) -> i32;