Fix leak of session->skip_rules

The list skip_rules is allocated in xccdf_session_new_from_source,
but was not freed in xccdf_session_free.

Author: 0intro

src/XCCDF/xccdf_session.c

@@ -358,6 +358,7 @@ void xccdf_session_free(struct xccdf_session *session)
 	oscap_htable_free(session->oval.arf_report_mapping, (oscap_destruct_func) free);
 	oscap_signature_ctx_free(session->signature_ctx);
 	oscap_list_free(session->rules, (oscap_destruct_func) free);
+	oscap_list_free(session->skip_rules, (oscap_destruct_func) free);
 	free(session);
 }