CMakeLists: Link in zlib privately where applicable

Also, use the target name introduced via find_package(ZLIB) instead
of hardcoding the 'z' name in the link libraries.

Source/Core/DiscIO/CMakeLists.txt

@@ -21,3 +21,8 @@ add_library(discio
   WiiSaveBanner.cpp
   WiiWad.cpp
 )
+
+target_link_libraries(discio
+PRIVATE
+  ZLIB::ZLIB
+)