gsf-libxml: mark some functions G_GNUC_PRINTF

Mark these functions as accepting printf-style format strings, so that
when they pass their arguments on to g_logv() the compiler knows not to
complain.

https://bugzilla.gnome.org/show_bug.cgi?id=720088
Compilation: fix compilation after printf changes.
openpkg: fix fuzzed file crash.
Add Thai translation