support/scripts/mkusers: fix shellcheck error related to auto_id

SC2086 is now reported for auto_id since shellcheck 0.9.0:

In support/scripts/mkusers line 453:
                add_one_group "${g}" ${auto_id}
                                     ^--------^ SC2086 (info): Double quote to prevent globbing and word splitting.

So quote it to get rid of this error.

Signed-off-by: Romain Naour <romain.naour@smile.fr>
[Peter: quote variable instead of disabling check]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/scripts/mkusers

@@ -450,7 +450,7 @@ main() {
                 auto_id=${AUTO_USER_ID}
             fi
             for g in ${groups//,/ }; do
-                add_one_group "${g}" ${auto_id}
+                add_one_group "${g}" "${auto_id}"
             done
         fi
     done