LibWeb: Add Implement GainNode interface

As with all other current audio nodes we still need to wire up the
inputs and outputs so it can be properly used in an audio context - but
this is enough to implement the public IDL interface.

Userland/Libraries/LibWeb/Forward.h

@@ -700,6 +700,7 @@ class AudioParam;
 class AudioScheduledSourceNode;
 class BaseAudioContext;
 class DynamicsCompressorNode;
+class GainNode;
 class OfflineAudioContext;
 class OscillatorNode;
 class PeriodicWave;