[JENKINS-13035] Only show this option when more than 1

MyViewTabBar available.

core/src/main/resources/hudson/views/MyViewsTabBar/GlobalConfigurationImpl/config.groovy

 package hudson.views.MyViewsTabBar.GlobalConfigurationImpl
 
-import hudson.views.ViewsTabBar
+import hudson.views.MyViewsTabBar
 
 def f=namespace(lib.FormTagLib)
 
-def all = ViewsTabBar.all()
+def all = MyViewsTabBar.all()
 if (all.size()>1) {
     f.dropdownDescriptorSelector(title:_("My Views Tab Bar"),field:"myViewsTabBar")
 }