the qt headers live in the source dir in this configuration.

instead of hard-coding the path in the project file, use the correct
module variable. this requires harfbuzz-ng to be built after corelib.

Change-Id: If1b64b59a0939d3b5190331fcf70da566d4eeedd
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>