windows: add proper header for hweight64()

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/os/os-windows.h b/os/os-windows.h
index 7bfe3d2..d465818 100644
--- a/os/os-windows.h
+++ b/os/os-windows.h
@@ -15,6 +15,7 @@
 #include "../smalloc.h"
 #include "../file.h"
 #include "../log.h"
+#include "../lib/hweight.h"
 
 #include "windows/posix.h"