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/globals.h b/src/globals.h
index 3e61545..992ce2a 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -545,11 +545,11 @@
 #endif
 
 #ifdef FEAT_CLIPBOARD
-EXTERN VimClipboard clip_star;	/* PRIMARY selection in X11 */
+EXTERN Clipboard_T clip_star;	// PRIMARY selection in X11
 # ifdef FEAT_X11
-EXTERN VimClipboard clip_plus;	/* CLIPBOARD selection in X11 */
+EXTERN Clipboard_T clip_plus;	// CLIPBOARD selection in X11
 # else
-#  define clip_plus clip_star	/* there is only one clipboard */
+#  define clip_plus clip_star	// there is only one clipboard
 #  define ONE_CLIPBOARD
 # endif