net/opennx: Fix build on armv7

time_t is a long long on armv7, but OpenNX tries to store time stamps
into configuration storage that only eats long variables. Cast to long
and cross fingers that nobody is going to use this port past 2036.

PR: 267618
Approved by: portmgr (blanket: build fix)
MFH: 2022Q4

(cherry picked from commit dceaf660a726b36ac871827a7bbb70b89930f049)