explicit declaration of internal libxklavier functions

from lingnomekbd:
https://git.gnome.org/browse/libgnomekbd/commit/?id=18332fedc6a24e4efeecfdc78a006140c667568a
https://bugzilla.gnome.org/show_bug.cgi?id=643754
Merge pull request #12 from infirit/1.6

explicit declaration of internal libxklavier functions