Fix the build of qt-freetype with -system-zlib

The static library needs to link to zlib too, so that the plugins
using this library link to zlib.

At runtime this doesn't make a difference because -system-zlib means
QtCore is linked to zlib and the system zlib shared library was
loaded, so the symbols are resolved. However, since -Wl,-no-undefined
was added to the build, the plugin must link to zlib explicitly too.

Change-Id: I10d6df8814e823079fee76ae6a7d5b55057c9daf
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>

src/platformsupport/fontdatabases/basic/basic.pri

@@ -75,6 +75,7 @@ contains(QT_CONFIG, freetype) {
    DEFINES += FT2_BUILD_LIBRARY
    contains(QT_CONFIG, system-zlib) {
         DEFINES += FT_CONFIG_OPTION_SYSTEM_ZLIB
+        include($$PWD/../../../3rdparty/zlib_dependency.pri)
    }
 
 } else:contains(QT_CONFIG, system-freetype) {