Fix MplCpp test build for MSVC

The boost includes are not in a system location.
MSVC seems to generate an ambiguity when using
boost::python::object() directly in a constructor.

qt/widgets/mplcpp/CMakeLists.txt

@@ -37,6 +37,7 @@ mtd_add_qt_library (TARGET_NAME MantidQtWidgetsMplCpp
   INCLUDE_DIRS
       inc
       ../../../Framework/PythonInterface/core/inc
+      ${Boost_INCLUDE_DIRS}
       ${PYTHON_INCLUDE_PATH}
       ${PYTHON_NUMPY_INCLUDE_DIR}
   LINK_LIBS

qt/widgets/mplcpp/test/ArtistTest.h

@@ -29,7 +29,8 @@ public:
   // ----------------- failure tests ---------------------
 
   void testConstructWithNonArtistThrowsInvalidArgument() {
-    TS_ASSERT_THROWS(Artist(Python::Object()), std::invalid_argument);
+    Python::Object none;
+    TS_ASSERT_THROWS(Artist artist(none), std::invalid_argument);
   }
 };