depends: Enable unicode support on dbd for Windows

Summary:

define UNICODE and _UNICODE while compiling for Windows. This would make
dbd read filename as utf8 string.

Backport of core PR13884.

Test Plan: Run the Windows Gitian builds.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5604