win32: prototype _glib_get_dll_directory() and _glib_get_locale_dir()

Rather than using "extern" declarations of these win32 functions
everywhere they're needed, just prototype them in glib-private.h.
(Which also fixes the fact that they weren't prototyped in the files
where they're defined.)

https://bugzilla.gnome.org/show_bug.cgi?id=688109

glib/ggettext.c

@@ -28,6 +28,7 @@
 
 #include "ggettext.h"
 #include "glibintl.h"
+#include "glib-private.h"
 
 #include "galloca.h"
 #include "gthread.h"