patch 8.2.3788: lambda for option that is a function may be freed

Problem:    Lambda for option that is a function may be garbage collected.
Solution:   Set a reference in the funcref. (Yegappan Lakshmanan,
            closes #9330)
diff --git a/src/proto/gui_xim.pro b/src/proto/gui_xim.pro
index 60f30d7..8c5c53a 100644
--- a/src/proto/gui_xim.pro
+++ b/src/proto/gui_xim.pro
@@ -2,6 +2,7 @@
 int set_imactivatefunc_option(void);
 int set_imstatusfunc_option(void);
 void free_xim_stuff(void);
+int set_ref_in_im_funcs(int copyID);
 void im_set_active(int active);
 void xim_set_focus(int focus);
 void im_set_position(int row, int col);