string(TIMESTAMP): Fix %s placeholder for year 2038 on Windows

`difftime` returns a `double`, so commit 6727270b75 (CMake: Extend
TIMESTAMP sub-commands with new unix time format specifier, 2016-02-16,
v3.6.0-rc1~338^2) cast it to `long` to get an integer result.  However,
in the MSVC ABI, `long` is a 32-bit integer.  Use an explicit 64-bit
integer type instead.

Fixes: #26716