patch 8.1.1076: file for Insert mode is much too big

Problem:    File for Insert mode is much too big.
Solution:   Split off the code for Insert completion. (Yegappan Lakshmanan,
            closes #4044)
diff --git a/src/proto.h b/src/proto.h
index 4a5977f..8b299ec 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -89,6 +89,9 @@
 # include "hardcopy.pro"
 # include "hashtab.pro"
 # include "indent.pro"
+# ifdef FEAT_INS_EXPAND
+# include "insexpand.pro"
+# endif
 # include "json.pro"
 # include "list.pro"
 # include "blob.pro"