zink: prefer vulkan_core.h over vulkan.h

zink: prefer vulkan_core.h over vulkan.h

There's just two places where we need any of the WSI specific vulkan
includes, the rest of Zink should do just fine with vulkan_core.h. So
let's include the win32-specific header explicitly in those two places,
and reduce the need for WSI specifics inside zink itself. Kopper
handles the rest of the WSI integration.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21441>