input_engine: std::move engine name where applicable

We can allow the name to be moved into, allowing allocations to be
avoided.

src/input_common/input_engine.h

@@ -102,7 +102,7 @@ struct InputIdentifier {
 
 class InputEngine {
 public:
-    explicit InputEngine(const std::string& input_engine_) : input_engine(input_engine_) {}
+    explicit InputEngine(std::string input_engine_) : input_engine{std::move(input_engine_)} {}
 
     virtual ~InputEngine() = default;