support: fix typo in mkusers

This fixes the spurious "[: too many arguments" errors from mkusers.

Signed-off-by: Philip Paeps <philip@paeps.cx>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>