As the pax-web-jetty bundle doesn't support a dynamic import of "third-party" packages you'll need to create an extra fragment bundle that contains this class and attach it to the pax-web-jetty bundle. A working sample on how to add this to pax-web-jetty can be found at the pax web samples
In short the fragment bundle manifest needs to contain a Fragment host. With the felix-bundle-plugin it needs to look like this:
<Fragment-Host>org.ops4j.pax.web.pax-web-jetty</Fragment-Host>