net-mgmt/cacti88-spine: fix build

net-snmp-config --cflags returns some bogus cflags, including some
dreaded -Werror flags (?!), preventing the build. Filter those out
as a stop gap measure.

PR: 293905
MFH: 2026Q1
Approved by: portmgr (build fix blanket)
Sponsored by: Raptor Computing Systems, LLC