perl-net-ssh2: add support for libgcrypt crypto backend

perl-net-ssh2: add support for libgcrypt crypto backend

Fix usage of libgcrypt as crypto backend to libssh2, when building
perl-net-ssh2. In order to achieve that, we need to use 'depends on' the
libssh2 backends, which means the user will first need to enable libssh2 and
one of the supported backends, before being able to enable perl-net-ssh2.

Fixes
http://autobuild.buildroot.net/results/6ee18e7dd17f168c52f79e49cb5e94cf3aa3df1a/

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>