Fix Win32 symbol export lists for BIO functions: Added BIO_get_ex_new_index,

BIO_get_ex_num, BIO_get_ex_data and BIO_set_ex_data to ms/libeay{16,32}.def.
I'm not a Win32 hacker, but I think I've done it correctly.

Steve or Ben: can you confirm that it's correct?
              I don't want to break any Win32 stuff.