fix: the build with feature 'socket' on macOS (#2514)

Author: SteveLauC

src/sys/socket/sockopt.rs

@@ -1697,12 +1697,14 @@ impl<'a> Set<'a, OsString> for SetOsString<'a> {
 
 /// Getter for a `CString` value.
 #[cfg(apple_targets)]
+#[cfg(feature = "net")]
 struct GetCString<T: AsMut<[u8]>> {
     len: socklen_t,
     val: MaybeUninit<T>,
 }
 
 #[cfg(apple_targets)]
+#[cfg(feature = "net")]
 impl<T: AsMut<[u8]>> Get<CString> for GetCString<T> {
     fn uninit() -> Self {
         GetCString {