[LyX/master] Revert unintended change

[Richard Kimberly Heck]

commit 2165855d5e8596c20a8efa5d17040e5a0a47eebe
Author: Richard Kimberly Heck <rikiheck at lyx.org>
Date:   Thu Feb 27 14:36:46 2020 -0500

    Revert unintended change
---
 src/TocBackend.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/src/TocBackend.h b/src/TocBackend.h
index c6f1d38..026f4f8 100644
--- a/src/TocBackend.h
+++ b/src/TocBackend.h
@@ -149,7 +149,7 @@ private:
 	///
 	TocList tocs_;
 	///
-	std::map<std::string, std::shared_ptr<TocBuilder>> builders_;
+    std::map<std::string, std::unique_ptr<TocBuilder>> builders_;
 	/// Stores localised outliner names from this buffer and its children
 	std::map<std::string, docstring> outliner_names_;
 	///