When a library is examined, an object file within it can be left out
of the link if it does not provide symbols that the symbol table
needs.  Introducing `isc_stdtime_tostring` caused a build failure for
`update_test` because it now requires `libisc.a(stdtime.o)` and that
also exports the `isc_stdtime_get` symbol, meaning we have a
multiple definition error.

Add a local version of `isc_stdtime_tostring`, so that the linker
will not search for it in available object files.