Fix cpu_count() on some platforms

When we use sysconf(_SC_NPROCESSORS_ONLN) to get number of
cores, we have to include sysconf library that is unistd.h

Sometimes it happens to work just becuase unistd.h indirectly
included by some other libraries, but not always.

Reported and fixed by Eyal BD

No functional change.

src/platform.h

@@ -40,6 +40,7 @@ typedef unsigned __int64 uint64_t;
 
 #else
 #  include <inttypes.h>
+#  include <unistd.h>  // Used by sysconf(_SC_NPROCESSORS_ONLN)
 #endif
 
 #if !defined(_WIN32) && !defined(_WIN64) // Linux - Unix