Fix Linux compilation issue

I got some reports of having difficulties compiling HaloRay without
adding an include for QAbstractItemModel because of the checkIndex call.
QAbstractItemModel is a grandparent class for CrystalModel, and I have
been able to succesfully compile everything without the explicit
include, but it does not hurt to have it either.

src/gui/crystalModel.cpp

@@ -1,4 +1,5 @@
 #include "crystalModel.h"
+#include <QAbstractItemModel>
 
 CrystalModel::CrystalModel(std::shared_ptr<HaloSim::CrystalPopulationRepository> crystalRepository, QWidget *parent)
     : QAbstractTableModel(parent),