no longer add ITK to global path in exported CMake

It turns out that we don't need `include(${ITK_USE_FILE})` in
STIRConfig.cmake. This therefore avoids adding the ITK include directory
to the global include path.

Effectively fixes #432 as when building STIR, we don't mind (currently
anyway).

src/cmake/STIRConfig.cmake.in

@@ -79,7 +79,6 @@ if (@ITK_FOUND@)
   if(NOT ITK_FOUND)
     SET(STIR_FOUND OFF)
   endif()
-  include(${ITK_USE_FILE})
   set(STIR_BUILT_WITH_ITK TRUE)
 endif()