snmp_pf: address Coverity remarks about time_t

On most platforms time_t is 64 bits wide, but we assign it to 32 bits
variables. Unfortunately these are SNMP fields that are defined to be 32 bits
wide, so we cannot change this. We are similarly unable to prevent the passage
of time, meaning that time_t must remain 64 bits wide as well.

Explicitly cast to tell Coverity that we're aware of these limitations and do
not wish to be reminded of them.

CID: 1553869
CID: 1557025
CID: 1592232
CID: 1592234
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