refact: suppress warns on macos (#12449)

Signed-off-by: fufesou <linlong1266@gmail.com>

src/flutter.rs

@@ -15,11 +15,11 @@ use hbb_common::{
 };
 use serde::Serialize;
 use serde_json::json;
-
+#[cfg(target_os = "windows")]
+use std::io::{Error as IoError, ErrorKind as IoErrorKind};
 use std::{
     collections::{HashMap, HashSet},
     ffi::CString,
-    io::{Error as IoError, ErrorKind as IoErrorKind},
     os::raw::{c_char, c_int, c_void},
     str::FromStr,
     sync::{
@@ -111,6 +111,7 @@ pub extern "C" fn rustdesk_core_main() -> bool {
         #[cfg(target_os = "macos")]
         std::process::exit(0);
     }
+    #[cfg(not(target_os = "macos"))]
     false
 }