demos: Fix builds against Visual Studio headers

demos: Fix builds against Visual Studio headers

According to [1], '_timezone' is already used for a global variable in the
time.h system header that is supplied by Microsoft, so using that for our
variable name when we are including time.h either directly or indirectly
will cause trouble.

This renames such variables to '_tz' to avoid that

[1]: https://docs.microsoft.com/en-us/cpp/c-runtime-library/daylight-dstbias-timezone-and-tzname