package/sslh: systemd needs libconfig

systemd-sslh-generator unconditionally uses libconfig

Fixes:
 - http://autobuild.buildroot.org/results/d46581842906f7780f4a3a1ae21963e78e7f5276

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr:
  - select libconfig
  - drop changes in the .mk
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
1 file changed