WIN32: fix comparison of different sizes into compute_cpu_set()

[carlo-bramini]

Compiling with MinGW for x86_64/Aarch64 prints this warning:

libs/fiber/src/numa/windows/topology.cpp: In function ‘std::set<unsigned int> {anonymous}::compute_cpu_set(WORD, KAFFINITY)’:
libs/fiber/src/numa/windows/topology.cpp:93:28: warning: comparison of integer expressions of different signedness: ‘int’ and ‘long long unsigned int’ [-Wsign-compare]
   93 |         for ( int i = 0; i < sizeof( mask) * 8; ++i) {
      |                          ~~^~~~~~~~~~~~~~~~~~~

This is correct because sizeof() returns size_t, but hopefully it can be fixed with a tiny change, by simply replacing int with size_t.
This is not dangerous, however it's still useful for removing this annoying warning when compiling, if possible.