modules: avoid repeating the same include dirs over and over.

Similarly to the _LIBRARIES above, just add stuff that is not already
there.

modules/FindMagnum.cmake

@@ -506,7 +506,7 @@ foreach(component ${Magnum_FIND_COMPONENTS})
         string(TOUPPER ${dependency} _DEPENDENCY)
         if(MAGNUM_${_DEPENDENCY}_LIBRARY)
             list(APPEND _MAGNUM_${_COMPONENT}_DEPENDENCY_LIBRARIES ${MAGNUM_${_DEPENDENCY}_LIBRARY} ${_MAGNUM_${_DEPENDENCY}_LIBRARIES})
-            list(APPEND _MAGNUM_${_COMPONENT}_DEPENDENCY_INCLUDE_DIRS ${MAGNUM_${_DEPENDENCY}_INCLUDE_DIRS})
+            list(APPEND _MAGNUM_${_COMPONENT}_DEPENDENCY_INCLUDE_DIRS ${_MAGNUM_${_DEPENDENCY}_INCLUDE_DIRS})
         else()
             unset(MAGNUM_${_COMPONENT}_LIBRARY)
         endif()