toolchain: fix 3.16 selection for custom version

Fix d'oh typo from 911516f1 that
prevents custom version string 3.16 headers from working properly.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>