systemd: assign user exec attribute to systemd --user instances

Signed-off-by: Kenton Groombridge <me@concord.sh>

policy/modules/system/systemd.if

@@ -42,6 +42,7 @@ template(`systemd_role_template',`
 	type $1_systemd_t, systemd_user_session_type, systemd_log_parse_env_type;
 	init_pgm_spec_user_daemon_domain($1_systemd_t)
 	domain_user_exemption_target($1_systemd_t)
+	userdom_spec_user_application_exec_domain($1_systemd_t, $1)
 	ubac_constrained($1_systemd_t)
 	role $2 types $1_systemd_t;