The gossiper reads peer features from system keyspace. Also the snitch
code needs system keyspace, and since now it gets all its dependencies
from gossiper (will be fixed some day, but not now), it will do the same
for sys.ks.. Thus it's worth having gossiper->system_keyspace explicit
dependency.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>