meson: correct windows-version define
The macro "_WINVER" does nothing, the macro definitions that matter for windows API version selection are "_WIN32_WINNT" and "WINVER".
The header "sdkddkver.h" (which is included from thousands of different windows-headers) defines "WINVER" to the same value as "_WIN32_WINNT" of only the latter is defined, which explains why this works right now. But we shouldn't depend on that kind of luck, and instead define the right maco.
Fixes: 3aee4627 ("meson: add windows compiler checks and libraries")