core/service: const and pure'ify service_restart_usec_next()

src/core/service.c

@@ -348,7 +348,7 @@ static void service_start_watchdog(Service *s) {
                 log_unit_warning_errno(UNIT(s), r, "Failed to install watchdog timer: %m");
 }
 
-usec_t service_restart_usec_next(Service *s) {
+usec_t service_restart_usec_next(const Service *s) {
         unsigned n_restarts_next;
 
         assert(s);

src/core/service.h

@@ -266,7 +266,7 @@ extern const UnitVTable service_vtable;
 int service_set_socket_fd(Service *s, int fd, struct Socket *socket, struct SocketPeer *peer, bool selinux_context_net);
 void service_release_socket_fd(Service *s);
 
-usec_t service_restart_usec_next(Service *s);
+usec_t service_restart_usec_next(const Service *s) _pure_;
 
 int service_determine_exec_selinux_label(Service *s, char **ret);