Fix ..Default::default() for struct Config

Previously the `depfile` option added a new private member to the
`Config` struct - a breaking change which many users complained about.
Making the field public, but hiding it from the documentation and
excluding it from `serde` reverts the breakage, while keeping the
depfile functionality.

CHANGES

@@ -1,3 +1,7 @@
+# unreleased
+
+      * Revert: The `Config` struct now has a private member.
+
 # 0.26.0
 
       * Fix swapping of `>>=` and `<<=` in constants.

src/bindgen/config.rs

@@ -1010,8 +1010,15 @@ pub struct Config {
     pub only_target_dependencies: bool,
     /// Configuration options specific to Cython.
     pub cython: CythonConfig,
+    #[doc(hidden)]
     #[serde(skip)]
-    pub(crate) config_path: Option<StdPathBuf>,
+    /// Internal field for tracking from which file the config was loaded.
+    ///
+    /// Users should not set this field explicitly. Making the field private
+    /// prevents users from filling the struct with `..Default::default()`,
+    /// and creating a new InternalConfig struct would require more breaking
+    /// changes to our public API.
+    pub config_path: Option<StdPathBuf>,
 }
 
 impl Default for Config {