patch 8.1.2127: the indent.c file is a bit big

Problem:    The indent.c file is a bit big.
Solution:   Move C-indent code a a new cindent.c file.  Move other
            indent-related code to indent.c. (Yegappan Lakshmanan,
            closes #5031)
diff --git a/src/proto.h b/src/proto.h
index 625e8aa..ad12159 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -67,6 +67,7 @@
 # include "bufwrite.pro"
 # include "change.pro"
 # include "charset.pro"
+# include "cindent.pro"
 # include "cmdexpand.pro"
 # include "cmdhist.pro"
 # include "if_cscope.pro"