Add uuid generation extension to sample_psql.eex.

lib/mix/tasks/sample_psql.eex

@@ -4,3 +4,4 @@ CREATE DATABASE pleroma_dev OWNER pleroma;
 --Extensions made by ecto.migrate that need superuser access
 CREATE EXTENSION IF NOT EXISTS citext;
 CREATE EXTENSION IF NOT EXISTS pg_trgm;
+CREATE EXTENSION IF NOT EXISTS "uuid-ossp";