make zlib_dependency auto-add QtCore as a private dep

our zlib header includes qglobal.h, so we need the qtcore include dirs,
and qtcore is also where the actual code is compiled into.

Change-Id: I09f530a1b4e6160438215a6d7223c0771ce94f05
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>