Make gtk_im_modules_init properly private

Putting non-exported symbols in public headers can
only lead to confusion and day drinking. Don't do it.

Fixes: #3676

gtk/gtkimmodule.h

@@ -22,8 +22,6 @@
 
 G_BEGIN_DECLS
 
-void          gtk_im_modules_init (void);
-
 #define GTK_IM_MODULE_EXTENSION_POINT_NAME "gtk-im-module"
 
 G_END_DECLS

gtk/gtkimmoduleprivate.h

@@ -24,6 +24,8 @@
 
 G_BEGIN_DECLS
 
+void          gtk_im_modules_init (void);
+
 void           gtk_im_module_ensure_extension_point  (void);
 GtkIMContext * _gtk_im_module_create                 (const char *context_id);
 const char   * _gtk_im_module_get_default_context_id (GdkDisplay *display);

gtk/gtkmain.c

@@ -129,7 +129,7 @@
 #include "gtkwindowprivate.h"
 #include "gtkwindowgroup.h"
 #include "gtkprintbackendprivate.h"
-#include "gtkimmodule.h"
+#include "gtkimmoduleprivate.h"
 #include "gtkroot.h"
 #include "gtknative.h"
 #include "gtkpopcountprivate.h"