Fix serde_conv doctest by gating it on "macros" feature
The test uses `serde_as` which is only available with the macros feature.
/// We want to serialize it as a `[u8; 3]`.
///
/// ```rust
+/// # #[cfg(feature = "macros")] {
+/// # use serde_derive::{Serialize, Deserialize};
+///
/// #[derive(Clone, Copy, Debug, PartialEq)]
/// struct Rgb {
/// red: u8,
/// // We can now use the `RgbAsArray` adapter with `serde_as`.
///
/// #[serde_with::serde_as]
-/// #[derive(Debug, PartialEq, serde::Serialize, serde::Deserialize)]
+/// #[derive(Debug, PartialEq, Serialize, Deserialize)]
/// struct Colors {
/// #[serde_as(as = "RgbAsArray")]
/// one_rgb: Rgb,
///
/// // The types generated by `serde_conv` is also compatible with serde's with attribute
///
-/// #[derive(Debug, PartialEq, serde::Serialize, serde::Deserialize)]
+/// #[derive(Debug, PartialEq, Serialize, Deserialize)]
/// struct ColorsWith {
/// #[serde(with = "RgbAsArray")]
/// rgb_with: Rgb,
///
/// assert_eq!(json, serde_json::to_value(&data).unwrap());
/// assert_eq!(data, serde_json::from_value(json).unwrap());
+/// # }
/// ```
#[macro_export]
macro_rules! serde_conv {
