Export glib_init via GLIB_PRIVATE_CALL

This will be used in the next commit to call glib_init from the
gobject constructor, to ensure proper constructor ordering with
non-GNU libc.

https://bugzilla.gnome.org/show_bug.cgi?id=756139

glib/glib-init.h

@@ -25,6 +25,7 @@
 extern GLogLevelFlags g_log_always_fatal;
 extern GLogLevelFlags g_log_msg_prefix;
 
+void glib_init (void);
 void g_quark_init (void);
 
 #ifdef G_OS_WIN32