make sure variable is declared

https://lgtm.com/projects/g/mrdoob/three.js/snapshot/88225c1a6d4bcdcf2ab408a972a8319ff38f4f53/files/examples/webgl_shaders_ocean.html?sort=name&dir=ASC&mode=heatmap&excluded=false#V143

examples/webgl_shaders_ocean.html

@@ -140,7 +140,7 @@
 
 				//
 
-				gui = new dat.GUI();
+				var gui = new dat.GUI();
 
 				gui.add( parameters, 'distortionScale', 0, 8, 0.1 );
 				gui.add( parameters, 'size', 0.1, 10, 0.1 );