[clang-tidy] pass by value where appropriate

Found with modernize-pass-by-value

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/db/VHelper.cxx

@@ -23,14 +23,15 @@
 #include "song/Filter.hxx"
 
 #include <algorithm>
+#include <utility>
 
 #include <assert.h>
 #include <stdlib.h>
 #include <string.h>
 
-DatabaseVisitorHelper::DatabaseVisitorHelper(const DatabaseSelection &_selection,
+DatabaseVisitorHelper::DatabaseVisitorHelper(DatabaseSelection _selection,
 					     VisitSong &visit_song) noexcept
-	:selection(_selection)
+	:selection(std::move(_selection))
 {
 	// TODO: apply URI and SongFilter
 	assert(selection.uri.empty());

src/db/VHelper.hxx

@@ -60,7 +60,7 @@ public:
 	 * @param visit_song the callback function passed to
 	 * Database::Visit(); may be replaced by this class
 	 */
-	DatabaseVisitorHelper(const DatabaseSelection &selection,
+	DatabaseVisitorHelper(DatabaseSelection selection,
 			      VisitSong &visit_song) noexcept;
 	~DatabaseVisitorHelper() noexcept;