Use an explicit path for stats_tree_dialog.cpp.

Change the entry for stats_tree_dialog.cpp in ui/qt/QtShark.pro to
have a relative "../qt/" path, so that Qt Creator can find it from its
build...-release temporary directory.

Change-Id: I8fa40e15a7fbb643d5565e216ce92f73afcd4871
Reviewed-on: https://code.wireshark.org/review/2340
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>