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/glib-compile-schemas.c

@@ -36,6 +36,10 @@
 #include "gvdb/gvdb-builder.h"
 #include "strinfo.c"
 
+#ifdef G_OS_WIN32
+#include "glib/glib-private.h"
+#endif
+
 static void
 strip_string (GString *string)
 {
@@ -2003,7 +2007,6 @@ main (int argc, char **argv)
   };
 
 #ifdef G_OS_WIN32
-  extern gchar *_glib_get_locale_dir (void);
   gchar *tmp;
 #endif