select: use correct SIZEOF_ constant

At least under Windows, there is no SIZEOF_LONG, so it evaluates to 0 even
though sizeof(int) == sizeof(long). This should probably have been
CURL_SIZEOF_LONG, but the type of timeout_ms changed from long to time_t
anyway.
This triggered MSVC warning C4668 about implicitly replacing undefined
macros with '0'.

Closes https://github.com/curl/curl/pull/1362