Add missing import Qt3D.Extras to wave example

Allows the example to run. Keyboard input not working yet.

Change-Id: I2c152ba6b15878e1bbaad865a7147b66a7911e50
Reviewed-by: Paul Lemire <paul.lemire@kdab.com>

examples/qt3d/wave/main.qml

@@ -52,6 +52,7 @@ import QtQuick 2.1 as QQ2
 import Qt3D.Core 2.0
 import Qt3D.Render 2.0
 import Qt3D.Input 2.0
+import Qt3D.Extras 2.0
 
 Entity {
     id: root