patch 8.1.1531: clipboard type name is inconsistent

Problem:    Clipboard type name is inconsistent.
Solution:   Rename VimClipboard to Clipboard_T.
diff --git a/src/proto/gui_x11.pro b/src/proto/gui_x11.pro
index 955f374..5dde37d 100644
--- a/src/proto/gui_x11.pro
+++ b/src/proto/gui_x11.pro
@@ -46,10 +46,10 @@
 void gui_mch_clear_all(void);
 void gui_mch_delete_lines(int row, int num_lines);
 void gui_mch_insert_lines(int row, int num_lines);
-void clip_mch_lose_selection(VimClipboard *cbd);
-int clip_mch_own_selection(VimClipboard *cbd);
-void clip_mch_request_selection(VimClipboard *cbd);
-void clip_mch_set_selection(VimClipboard *cbd);
+void clip_mch_lose_selection(Clipboard_T *cbd);
+int clip_mch_own_selection(Clipboard_T *cbd);
+void clip_mch_request_selection(Clipboard_T *cbd);
+void clip_mch_set_selection(Clipboard_T *cbd);
 void gui_mch_menu_grey(vimmenu_T *menu, int grey);
 void gui_mch_menu_hidden(vimmenu_T *menu, int hidden);
 void gui_mch_draw_menubar(void);