Fix build on Harmattan when cross-compiling

Fix build on Harmattan when cross-compiling

When configure detects the presence of font-config/freetype via pkg-config
_and_ we are cross-compiling for Harmattan, make sure that we pick up the
include path for freetype. The .pri file is included in various places in
qtbase when the usage of freetype is required.

Change-Id: I4a5390dfc063504def80a891bc9153d6a376ccd7
Reviewed-on: http://codereview.qt-project.org/6509
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>