build: add missing `Bcrypt` link on Windows
When building on Windows, we must link against Bcrypt to satisfy the
use of BCryptGenRandom. Add this to the link interface in CMake to
allow use of libxml2 as a static library.
When building on Windows, we must link against Bcrypt to satisfy the
use of BCryptGenRandom. Add this to the link interface in CMake to
allow use of libxml2 as a static library.