[CMAKE] Fix DLL exports

Summary:
Some symbols are missing from the libbitcoinconsensus DLL, because it
expects the preprocessor DLL_EXPORT symbol to be defined, but it is
defined by libtool only.

Test Plan:

cmake -GNinja .. \
  -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Win64.cmake \
  -DBUILD_BITCOIN_SEEDER=OFF
ninja

Use a DLL inspection tool to verify that the bitcoinconsensus_*
symbols are exported (checked with Dependency Walker
http://www.dependencywalker.com/ on windows).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4836