stunnel: use BR2_TOOLCHAIN_HAS_SSP instead of BR2_ENABLE_SSP

All packages use BR2_TOOLCHAIN_HAS_SSP to determine whether they can
use SSP suppport or not, except stunnel. Therefore, this commit makes
stunnel consistent with other packages. This is also necessary in
preparation to the removal of BR2_ENABLE_SSP in favor of a choice
between various SSP options.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>