grue example: Set qmldir.path correctly.

It should be $$DESTPATH, not $$DESTDIR, otherwise the build will
install files to wrong locations like debian/tmp/build/buildd/...
when building Debian packages.

Change-Id: Iae1bd2d190dd1653f06d0f043f1b701b7cb46eb4
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>