timedatectl: fix a memory leak

```
timedatectl list-timezones --no-pager
...
==164329==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 8192 byte(s) in 1 object(s) allocated from:
    #0 0x7fe8a74b6f8c in reallocarray (/lib64/libasan.so.6+0xaef8c)
    #1 0x7fe8a63485dc in strv_push ../src/basic/strv.c:419
    #2 0x7fe8a6349419 in strv_consume ../src/basic/strv.c:490
    #3 0x7fe8a634958d in strv_extend ../src/basic/strv.c:542
    #4 0x7fe8a643d787 in bus_message_read_strv_extend ../src/libsystemd/sd-bus/bus-message.c:5606
    #5 0x7fe8a643db9d in sd_bus_message_read_strv ../src/libsystemd/sd-bus/bus-message.c:5628
    #6 0x4085fb in list_timezones ../src/timedate/timedatectl.c:314
    #7 0x7fe8a61ef3e1 in dispatch_verb ../src/shared/verbs.c:103
    #8 0x410f91 in timedatectl_main ../src/timedate/timedatectl.c:1025
    #9 0x41111c in run ../src/timedate/timedatectl.c:1043
    #10 0x411242 in main ../src/timedate/timedatectl.c:1046
    #11 0x7fe8a489df1f in __libc_start_call_main (/lib64/libc.so.6+0x40f1f)
```

src/timedate/timedatectl.c

@@ -304,7 +304,7 @@ static int list_timezones(int argc, char **argv, void *userdata) {
         sd_bus *bus = userdata;
         _cleanup_(sd_bus_message_unrefp) sd_bus_message *reply = NULL;
         int r;
-        char** zones;
+        _cleanup_strv_free_ char **zones = NULL;
 
         r = bus_call_method(bus, bus_timedate, "ListTimezones", &error, &reply, NULL);
         if (r < 0)