fattime: make the test code build again

[jeffpc_josefsipek.net]

Summary

This change...

1. replaces calls to timet2fattime/fattime2timet with calls to timespec2fattime/fattime2timespec. The functions got renamed shortly after they landed in the kernel but the test code wasn't updated (see 7ea93e912bf0ef).
2. adds a utc_offset stub.

With this, the test code builds and runs as a 32-bit binary (cc -Wall -O2
-m32 subr_fattime.c).

[imp]

sys/kern/subr_fattime.c

301

why 1?

[jeffpc_josefsipek.net]

sys/kern/subr_fattime.c

301

I'm not happy with hardcoding 1 like this, but...

1. the original test code assumed UTC (the subsequent change that renamed the functions (and broke the test) also added the int utc arg) and using 1 keeps the test UTC-based (not that it matters since the stub for utc_offset() always returns 0)
2. the arg is int, so it seems wrong to use anything other than 0 or 1.