Make more xmlError structs constant

parameter-framework's upstream is not active anymore so we can't send
this patch upstream. github.com/GNOME/libxml2/commit/4547061 is causing
an error in parameter-framework.

Test: TreeHugger
Change-Id: Ib7b155f5b7bbbbe539e5e8939ccc53c4d178d09c
2 files changed