cmake: fix DEFINE_SYMBOL on windows

The `XDMF_EXPORTS` symbol was not getting defined correctly since it was
being set on the wrong target. This was causing windows builds to
failed. Fixed that.

CMakeLists.txt

@@ -353,7 +353,8 @@ if(NOT XDMF_BUILD_CORE_ONLY)
 
   if(WIN32)
     if (BUILD_SHARED_LIBS)
-      set_target_properties(${XDMF_LIBNAME} PROPERTIES
+      # XXX(kitware) using correct target name
+      set_target_properties(xdmf3 PROPERTIES
         DEFINE_SYMBOL XDMF_EXPORTS)
     endif ()
   endif()