dhcp: Use correct format specifiers in a message

dhcp: Use correct format specifiers in a message

Format size_t with %zu specifier, instead of %d, as size_t is not
guaranteed to be the same size as int.

This fixes the following compiler warnings:

freeradius-server-2.2.6/src/lib/dhcp.c: scope_hint: In function 'fr_dhcp_add_arp_entry'
freeradius-server-2.2.6/src/lib/dhcp.c:1536: warning: format '%d' expects type 'int', but argument 2 has type 'long unsigned int'
freeradius-server-2.2.6/src/lib/dhcp.c:1536: warning: format '%d' expects type 'int', but argument 3 has type 'size_t'