Make the schemars_0_8 module private

It has no members so there's no point in adding it to the public API.

serde_with/src/lib.rs

@@ -331,7 +331,7 @@ pub mod json;
 mod key_value_map;
 pub mod rust;
 #[cfg(feature = "schemars_0_8")]
-pub mod schemars_0_8;
+mod schemars_0_8;
 pub mod ser;
 #[cfg(feature = "std")]
 mod serde_conv;