Fix build gcc option -Wincompatible-pointer-types

It fails to build xfwm4 with gcc option -Wincompatible-pointer-types:

| ../../xfwm4-4.18.0/src/startup_notification.c:139:57: error: passing argument 2 of 'sn_startup_sequence_get_last_active_time' from incompatible pointer type [-Wincompatible-pointer-types]
|   139 |     sn_startup_sequence_get_last_active_time (sequence, &l_sec, &l_usec);
|       |                                                         ^~~~~~
|       |                                                         |
|       |                                                         long int *
| In file included from /usr/include/startup-notification-1.0/libsn/sn.h:32,
|                  from ../../xfwm4-4.18.0/src/startup_notification.c:33:
| /usr/include/startup-notification-1.0/libsn/sn-monitor.h:84:79: note: expected 'time_t *' {aka 'long long int *'} but argument is of type 'long int *'
|    84 |                                                            time_t            *tv_sec,
|       |