Fix compile error in cpu_count()

Fix compile error in cpu_count()

The std::min() template function requires both arguments
to be of the same type.

But here we have the integer MAX_THREADS compared to a long:

long sysconf(int name);

So cast to integer and fix the compile.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>