patch 8.1.2077: the ops.c file is too big

Problem:    The ops.c file is too big.
Solution:   Move code for dealing with registers to a new file. (Yegappan
            Lakshmanan, closes #4982)
diff --git a/src/proto.h b/src/proto.h
index 0081ad0..ae4c63a 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -188,6 +188,7 @@
 # endif
 # include "quickfix.pro"
 # include "regexp.pro"
+# include "register.pro"
 # include "scriptfile.pro"
 # include "screen.pro"
 # include "session.pro"