make-ring: libgnutls-dev to libgnutls28-dev

In Debian and Ubuntu dependencies specify libgnutls28-dev as
libgnutls-dev doesn't exist in official repositories.

Change-Id: Ib151d56cca6541b3f5eac797d30d808dc54a4604
[stepan.salenikovich@savoirfairelinux.com: updated commit message;
 made same change for ubuntu]
Signed-off-by: Stepan Salenikovich <stepan.salenikovich@savoirfairelinux.com>
Reviewed-by: Stepan Salenikovich <stepan.salenikovich@savoirfairelinux.com>