audio/radiotrope: fix build on armv7
The usual issue of time_t being 64 bits on armv7 despite it being a
32 bit platform. Refactor the time conversion code to just use i64
throughout, avoiding any type conversion issues.
Approved by: portmgr (build fix blanket)
MFH: 2026Q2
(cherry picked from commit 0c9905d54dedcf8763df802a01beb2ea3d4ac4a0)