net-mgmt/cacti-spine: Fix build with net-snmp

net-snmp-config --cflags injects -Werror=declaration-after-statement
which causes builds to fail as the source has variable declarations
after statements (valid in C99 but not C89).

Downgrade this from a fatal error to a warning

PR: ports/292340
Reported by: Robert Blayzor <rblayzor@inoc.net>