(ITS#8271) win32: add cast to avoid warning about LONGLONG to size_t

[nacho.resa@gmail.com]

Full_Name: Ignacio Casal Quinteiro
Version: 
OS: Windows 7
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (2a01:111f:9fa:7f00:864b:f5ff:fe34:ddaa)


Patch can be found here.
https://github.com/nice-software/lmdb/commit/2030f7dca473e66bd66ebee759ecfe4e187cb0fc