filter/Observer: pass Reset() to underlying Filter

Wohooooo, the method Filter::Reset() has been broken because no
implementation of it has ever been called for a loooong time.
And nobody ever noticed it.  WTF.

src/filter/Observer.cxx

@@ -73,6 +73,10 @@ public:
 		return filter;
 	}
 
+	void Reset() override {
+		filter->Reset();
+	}
+
 	ConstBuffer<void> FilterPCM(ConstBuffer<void> src) override {
 		return filter->FilterPCM(src);
 	}