dhcp.cgi: Fix incorrect { placement from patch 3724

dhcp.cgi: Fix incorrect { placement from patch 3724

- When patch 3724 was created for bug #10743 a curly bracket was placed in the wrong place
This results in the overlap of two if loops meaning that there will be no validity
check carried out on Default Lease Time if Deny Known Clients is not checked.
- This patch moves the { bracket to the right location.

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>