Fix more unintended copies in range-based for loops

Spotted by g++ 11.

src/Encoding.cpp

@@ -267,7 +267,7 @@ vector<char_type> Encoding::symbolsList() const
 	// add all encodable characters
 	copy(encodable_.begin(), encodable_.end(), back_inserter(symbols));
 	// now the ones from the unicodesymbols file that are not already there
-	for (pair<char_type, CharInfo> const & elem : unicodesymbols) {
+	for (pair<const char_type, CharInfo> const & elem : unicodesymbols) {
 		if (find(symbols.begin(), symbols.end(), elem.first) == symbols.end())
 			symbols.push_back(elem.first);
 	}

src/TocBackend.cpp

@@ -272,7 +272,7 @@ void TocBackend::resetOutlinerNames()
 {
 	outliner_names_.clear();
 	// names from this document class
-	for (pair<string, docstring> const & name
+	for (pair<const string, docstring> const & name
 		     : buffer_->params().documentClass().outlinerNames())
 		addName(name.first, translateIfPossible(name.second));
 	// Hardcoded types

src/frontends/qt4/TocModel.cpp

@@ -353,7 +353,7 @@ void TocModels::reset(BufferView const * bv)
 	names_->insertColumns(0, 1);
 	// In the outliner, add Tocs from the master document
 	TocBackend const & backend = bv->buffer().masterBuffer()->tocBackend();
-	for (pair<string, shared_ptr<Toc>> const & toc : backend.tocs()) {
+	for (pair<const string, shared_ptr<Toc>> const & toc : backend.tocs()) {
 		QString const type = toqstr(toc.first);
 
 		// First, fill in the toc models.

src/insets/InsetInclude.cpp

@@ -1288,7 +1288,7 @@ void InsetInclude::addToToc(DocIterator const & cpit, bool output_active,
 		//Copy missing outliner names (though the user has been warned against
 		//having different document class and module selection between master
 		//and child).
-		for (pair<string, docstring> const & name
+		for (pair<const string, docstring> const & name
 			     : childbuffer->params().documentClass().outlinerNames())
 			backend.addName(name.first, translateIfPossible(name.second));
 	}