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

gio/gresource-tool.c

@@ -40,6 +40,10 @@
 #include <glib/gstdio.h>
 #include <gi18n.h>
 
+#ifdef G_OS_WIN32
+#include "glib/glib-private.h"
+#endif
+
 /* GResource functions {{{1 */
 static GResource *
 get_resource (const gchar *file)
@@ -583,7 +587,6 @@ main (int argc, char *argv[])
   void (* function) (const gchar *, const gchar *, const gchar *, gboolean);
 
 #ifdef G_OS_WIN32
-  extern gchar *_glib_get_locale_dir (void);
   gchar *tmp;
 #endif