nspawn: unref the notify event source (#4941)

nspawn: unref the notify event source (#4941)

Fixes:
```
sudo ./libtool --mode=execute valgrind --leak-check=full ./systemd-nspawn -D ./CONT/ -b
...
==21224== 2,444 (656 direct, 1,788 indirect) bytes in 1 blocks are definitely lost in loss record 13 of 15
==21224==    at 0x4C2FA50: calloc (vg_replace_malloc.c:711)
==21224==    by 0x4F6F565: sd_event_new (sd-event.c:431)
==21224==    by 0x1210BE: run (nspawn.c:3351)
==21224==    by 0x123908: main (nspawn.c:3826)
==21224==
==21224== LEAK SUMMARY:
==21224==    definitely lost: 656 bytes in 1 blocks
==21224==    indirectly lost: 1,788 bytes in 11 blocks
==21224==      possibly lost: 0 bytes in 0 blocks
==21224==    still reachable: 8,344 bytes in 3 blocks
==21224==         suppressed: 0 bytes in 0 blocks
```
Closes #4934