Pass through cfg attrs

Otherwise things like cfg(target_os = "android") cannot be used
within a cxx bridge.

Resolves error:
error: unsupported attribute
 --> system/bt/gd/rust/common/src/sys_prop.rs:4:1
  |
4 | #[cfg(target_os = "android")]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

When updating to cxx version 1.0.x in AOSP.
https://cs.android.com/android/platform/superproject/+/master:system/bt/gd/rust/common/src/sys_prop.rs;l=3

syntax/attrs.rs

@@ -131,6 +131,10 @@ pub fn parse(cx: &mut Errors, attrs: Vec<Attribute>, mut parser: Parser) -> Othe
             // https://doc.rust-lang.org/reference/attributes/diagnostics.html
             passthrough_attrs.push(attr);
             continue;
+        } else if attr.path.is_ident("cfg")
+        {
+            passthrough_attrs.push(attr);
+            continue;
         } else if attr.path.segments.len() > 1 {
             let tool = &attr.path.segments.first().unwrap().ident;
             if tool == "rustfmt" {