GetPrerequisites: fix regression in gp_append_unique

The `item` variable was the loop variable in the old code prior to
commit 3dfc8b9bd6 (GetPrerequisites: use if(IN_LIST) instead of foreach,
2019-05-09).  The function argument `value` should be used instead.

Modules/GetPrerequisites.cmake

@@ -173,7 +173,7 @@ cmake_policy(PUSH)
 cmake_policy(SET CMP0057 NEW) # if IN_LIST
 
 function(gp_append_unique list_var value)
-  if(NOT item IN_LIST ${list_var})
+  if(NOT value IN_LIST ${list_var})
     set(${list_var} ${${list_var}} "${value}" PARENT_SCOPE)
   endif()
 endfunction()