patch 8.1.1869: code for the argument list is spread out

Problem:    Code for the argument list is spread out.
Solution:   Put argument list code in arglist.c. (Yegappan Lakshmanan,
            closes #4819)
diff --git a/src/proto.h b/src/proto.h
index f6f5319..ab0bee3 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -61,6 +61,7 @@
 #  include "crypt.pro"
 #  include "crypt_zip.pro"
 # endif
+# include "arglist.pro"
 # include "autocmd.pro"
 # include "buffer.pro"
 # include "change.pro"