Explicitly include boost header

Co-authored-by: James Clarke <139879523+jclarkeSTFC@users.noreply.github.com>

Author: jhaigh0

Framework/PythonInterface/mantid/api/src/Exports/SpectrumInfoPythonIterator.cpp

@@ -9,6 +9,7 @@
 #include <boost/python/class.hpp>
 #include <boost/python/iterator.hpp>
 #include <boost/python/module.hpp>
+#include <boost/python/reference_existing_object.hpp>
 
 using Mantid::PythonInterface::SpectrumInfoPythonIterator;
 using namespace boost::python;