remove compilation error messages (As noted by Alan Buxey)

remove compilation error messages (As noted by Alan Buxey)

removes the following at compile time:

dhcp.c: In function ‘fr_dhcp_add_arp_entry’:
dhcp.c:1561: warning: format ‘%lu’ expects type ‘long unsigned int’,
but argument 2 has type ‘unsigned int’
dhcp.c:1561: warning: format ‘%lu’ expects type ‘long unsigned int’,
but argument 3 has type ‘size_t’