winbuild: conditionally use /DZLIB_WINAPI

zlibwapi.lib (dynamic library) and zlibstat.lib (static library) have
the ZLIB_WINAPI define set by default. Using them requires that define
too.

Ref: https://zlib.net/DLL_FAQ.txt

Fixes https://github.com/curl/curl/issues/3133
Closes https://github.com/curl/curl/pull/3460