vcl: Make ImageTree ctor private

There's only a single instance of the class
that can be retrieved using ImageTree::get.

Make the ctor private to clarify that.

Change-Id: I75bd6e60e8079f5ddcef49200280fa61bbf1c4ca
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/211552
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
Tested-by: Jenkins

include/vcl/ImageTree.hxx

@@ -56,9 +56,9 @@ class ImageTree
 private:
     std::unique_ptr<ImplImageTree> mpImplImageTree;
 
-public:
     ImageTree();
 
+public:
     VCL_DLLPUBLIC static ImageTree & get();
 
     VCL_DLLPUBLIC OUString getImageUrl(