COMP: Fix compilation of Segment subject hierarchy plugin with python disabled

This commit will fix the following error:

//--------------
[ 54%] Building CXX object Modules/Loadable/SubjectHierarchy/Widgets/CMakeFiles/qSlicerSubjectHierarchyModuleWidgets.dir/qSlicerSubjectHierarchySegmentPlugin.cxx.o
/Users/essert/DEV/Src/Slicer/Modules/Loadable/SubjectHierarchy/Widgets/qSlicerSubjectHierarchySegmentPlugin.cxx:59:10: fatal error: 'PythonQt.h' file
not found
#include "PythonQt.h"
^
1 error generated.
//--------------

Reported-by: Caroline Essert <essert@unistra.fr>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24364 3bd1e089-480b-0410-8dfb-8563597acbee