loader: userboot: provide a getsecs() implementation

We don't need it for userboot, but it avoids issues with BIND_NOW, so
just provide it. time(3) isn't defined but ends up being provided by
libc linked into the host process, which is generally fine.

PR: 262920
Reviewed by: imp, jhb

(cherry picked from commit 660c1892d5c90500d37f98185326c6287b2b61be)