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/os_unix.pro b/src/proto/os_unix.pro
index 0e7c609..449b3e9 100644
--- a/src/proto/os_unix.pro
+++ b/src/proto/os_unix.pro
@@ -79,10 +79,10 @@
 void start_xterm_trace(int button);
 void stop_xterm_trace(void);
 void clear_xterm_clip(void);
-int clip_xterm_own_selection(VimClipboard *cbd);
-void clip_xterm_lose_selection(VimClipboard *cbd);
-void clip_xterm_request_selection(VimClipboard *cbd);
-void clip_xterm_set_selection(VimClipboard *cbd);
+int clip_xterm_own_selection(Clipboard_T *cbd);
+void clip_xterm_lose_selection(Clipboard_T *cbd);
+void clip_xterm_request_selection(Clipboard_T *cbd);
+void clip_xterm_set_selection(Clipboard_T *cbd);
 int xsmp_handle_requests(void);
 void xsmp_init(void);
 void xsmp_close(void);