doc: update libsso link for SNOW 3G

doc: update libsso link for SNOW 3G

A new process to request the libsso library required by the SNOW3G PMD
has been put in place, through a website, replacing the previous email method.
This commit updates the SNOW3G documentation, to reflect this change.

Since the library does not support newer gcc versions, the documentation
also contains a patch to make the library work with gcc > 5.0.

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Declan Doherty <declan.doherty@intel.com>