system-monitor: Unbreak runtime on (at least) BSD

Calling glibtop_init is not optional on BSD[0], and not doing so will
crash gnome-shell with:

  glibtop(c=57582): [ERROR] glibtop_get_swap: Function not implemented
  gnome-session-binary[34511]: WARNING: Application 'org.gnome.Shell.desktop' killed by signal 6

[0]: https://gitlab.gnome.org/GNOME/libgtop/-/issues/44

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/356>

extensions/system-monitor/extension.js

@@ -361,6 +361,7 @@ class Indicator extends PanelMenu.Button {
     }
 
     constructor(settings) {
+        GTop.glibtop_init();
         super(0.5, _('System stats'));
 
         this._settings = settings;