systemd-journal: fix spurious warning about about incorrect size passed to strncpy()

[bazsi]

    .../modules/systemd-journal/journal-reader.c: In function '_get_value_from_message.constprop.isra':
    .../modules/systemd-journal/journal-reader.c:218:7: warning: '__builtin___strncpy_chk' output truncated before terminating nul copying as many bytes from a string as its length [-Wstringop-truncation]
      218 |       strncpy(buf + cont, key, key_len);
          |       ^
    In function '_format_value_name_with_prefix',
        inlined from '_get_value_from_message.constprop.isra' at /source/modules/systemd-journal/journal-reader.c:243:3:
    .../modules/systemd-journal/journal-reader.c:211:15: note: length computed here
      211 |     key_len = strlen(key);
          |               ^~~~~~~~~~~

This wasn't an actual issue as we were explicitly terminating the string with NUL in this case, so missing the NUL character was actually expected.