net-mgmt/net-snmp: Fix ports depending on this port under 15-CURRENT

Also add freebsd15.h to ${LOCALBASE}/include/net-snmp/system, missing
in 710a64c4471d. ${LOCALBASE}/include/net-snmp/system/freebsd15.h is
used by ports which depend on net-mgmt/net-snmp. Without this file
they also fail to build under 15-CURRENT.

Approved by: portmgr (blanket)
Fixes: 710a64c4471d