Move popcount() to an optionally built part of wsutil for systems that
provide their own popcount().

Change-Id: Ic26f3b50cf0bd2b4af0d42e9c27488ebbac1ab33
Reviewed-on: https://code.wireshark.org/review/5998
Petri-Dish: Stephen Fisher <sfisher@sdf.org>
Reviewed-by: Stephen Fisher <sfisher@sdf.org>