libsysprof: Kill dbus-monitor after D-Bus capturing is done

Previously, the `dbus-monitor` process was never terminated.
Unfortunately, tying up some memory is not the worst consequence
of this: after some time file descriptors that are sent on the
bus start to accumulate. This can be especially problematic
when there are unexpected read/write ends of pipes.

src/libsysprof/sysprof-dbus-monitor.c

@@ -216,6 +216,8 @@ sysprof_dbus_monitor_record_fiber (gpointer user_data)
         }
     }
 
+  g_subprocess_force_exit (subprocess);
+
   return dex_future_new_for_boolean (TRUE);
 }