Add missing #[inline] on AsFd impl for sys::unix::fs::File

This operation should be extremely cheap, at most the mov of the underlying
file descriptor, but due to this missing #[inline] it is currently a function
call.

library/std/src/sys/unix/fs.rs

@@ -1391,6 +1391,7 @@ impl FromInner<FileDesc> for File {
 }
 
 impl AsFd for File {
+    #[inline]
     fn as_fd(&self) -> BorrowedFd<'_> {
         self.0.as_fd()
     }