patch 8.2.0443: clipboard code is spread out

Problem:    Clipboard code is spread out.
Solution:   Move clipboard code to its own file. (Yegappan Lakshmanan,
            closes #5827)
diff --git a/src/proto.h b/src/proto.h
index 74f38cd..5d81557 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -68,6 +68,7 @@
 # include "change.pro"
 # include "charset.pro"
 # include "cindent.pro"
+# include "clipboard.pro"
 # include "cmdexpand.pro"
 # include "cmdhist.pro"
 # include "if_cscope.pro"