Fix a number of undeclared variable warnings

"for (prop in object)" needs a "var prop" before, or it declares
a global property.

extensions/alternate-tab/extension.js

@@ -54,6 +54,8 @@ function enable() {
 }
 
 function disable() {
+    var prop;
+
     setKeybinding('switch-applications', Lang.bind(Main.wm, Main.wm._startAppSwitcher));
     setKeybinding('switch-group', Lang.bind(Main.wm, Main.wm._startAppSwitcher));
     setKeybinding('switch-applications-backward', Lang.bind(Main.wm, Main.wm._startAppSwitcher));

extensions/native-window-placement/extension.js

@@ -480,6 +480,8 @@ function removeInjection(object, injection, name) {
 }
 
 function disable() {
+    var i;
+
     for (i in workspaceInjections)
         removeInjection(Workspace.Workspace.prototype, workspaceInjections, i);
     for (i in winInjections)

extensions/window-list/extension.js

@@ -842,6 +842,8 @@ function enable() {
 }
 
 function disable() {
+    var prop;
+
     if (!windowList)
         return;