systemd: initialize pinnedUnits state

This issue has been uncovered by adding a link to the logs page from
cockpit-podman. Once clicking on it the initial state is undefined and
the ServiceDetails page raises an exception:

> error: TypeError: Cannot read properties of undefined (reading 'includes')
> error: The above error occurred in the <ServiceDetails> component:

    at ServiceDetails (http://127.0.0.2:9691/cockpit/@localhost/system/services.js:38578:7)

Author: jelly

pkg/systemd/services.jsx

@@ -182,6 +182,7 @@ class ServicesPageBody extends React.Component {
             /* State related to the toolbar components */
             isFullyLoaded: false,
             error: null,
+            pinnedUnits: [],
         };
 
         /* data storage