package/pppd: requires DES support in openssl

Enable DES in openssl to fix build failure raised since commit
a83d4186

Fixes:
 - http://autobuild.buildroot.org/results/3cd38e6dfd5f94f0bd1689e7db4ea1d723dd2a80



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr: only select it for glibc toolchains]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>