daemon: use str_udevadm in udev_settle

There is GUESTFSD_EXT_CMD defining a string for udevadm (so it is marked
as "used tool" in the appliance), but it is not actually used when
starting udevadm.

There should be no behaviour change.

daemon/guestfsd.c

@@ -1501,7 +1501,8 @@ udev_settle (void)
   char cmd[80];
   int r;
 
-  snprintf (cmd, sizeof cmd, "udevadm%s settle", verbose ? " --debug" : "");
+  snprintf (cmd, sizeof cmd, "%s%s settle",
+            str_udevadm, verbose ? " --debug" : "");
   if (verbose)
     printf ("%s\n", cmd);
   r = system (cmd);