Compile fix: use Utils::optional instead of std::optional

Amends d9cd74f06d

Change-Id: Ic4a971fa2fb4659784afdc19f5b8db6a378f611e
Reviewed-by: hjk <hjk@qt.io>

src/plugins/clangformat/clangformatbaseindenter.h

@@ -66,7 +66,7 @@ public:
 
     bool isElectricCharacter(const QChar &ch) const override;
 
-    std::optional<int> margin() const override;
+    Utils::optional<int> margin() const override;
 
 protected:
     virtual clang::format::FormatStyle styleForFile() const;