Your terminology is a little confusing. By "Custom Control" I'm assuming you mean "Component" as a custom control contains XSP Markup inside it (i.e. XML). But to address your issue, in the MANIFEST.MF file of your plugin, on the dependencies tab in the required plug-ins. Add com.ibm.xsp.extlib as a dependency. While you're there you'll probably also need to add:
- com.ibm.xsp.designer
- com.ibm.xsp.domino
- com.ibm.icu
Looking at your previous question... I mirror Tim's suggestion of getting the XSP Starter Kit as that contains examples of custom components, resource providers, phase listeners, JS Library inclusion, etc.