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/os_unix.c b/src/os_unix.c
index 01f5f59..a15d660 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -7775,7 +7775,7 @@
 }
 
     int
-clip_xterm_own_selection(VimClipboard *cbd)
+clip_xterm_own_selection(Clipboard_T *cbd)
 {
     if (xterm_Shell != (Widget)0)
 	return clip_x11_own_selection(xterm_Shell, cbd);
@@ -7783,21 +7783,21 @@
 }
 
     void
-clip_xterm_lose_selection(VimClipboard *cbd)
+clip_xterm_lose_selection(Clipboard_T *cbd)
 {
     if (xterm_Shell != (Widget)0)
 	clip_x11_lose_selection(xterm_Shell, cbd);
 }
 
     void
-clip_xterm_request_selection(VimClipboard *cbd)
+clip_xterm_request_selection(Clipboard_T *cbd)
 {
     if (xterm_Shell != (Widget)0)
 	clip_x11_request_selection(xterm_Shell, xterm_dpy, cbd);
 }
 
     void
-clip_xterm_set_selection(VimClipboard *cbd)
+clip_xterm_set_selection(Clipboard_T *cbd)
 {
     clip_x11_set_selection(cbd);
 }