Add missing call to g_type_init()

The Glib docs state "Prior to any use of the type system, g_type_init() has to
be called".[1] To not do so can lead to segfaults.  The g_type system is
currently used by various "filters" that operate on uuencoded text, message
headers, etc.

[1] http://developer.gnome.org/gobject/2.28/gobject-Type-Information.html#g-type-init