Add yansi-term as a permitted dependency

Author: jyn514

src/tools/tidy/src/deps.rs

@@ -189,6 +189,8 @@ const PERMITTED_DEPENDENCIES: &[&str] = &[
     "winapi-i686-pc-windows-gnu",
     "winapi-util",
     "winapi-x86_64-pc-windows-gnu",
+    // this is a false-positive: it's only used by rustfmt, but because it's enabled through a feature, tidy thinks it's used by rustc as well.
+    "yansi-term",
 ];
 
 /// Dependency checks.