vim-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 vim/vim#4044)
7591bb39d5

Cherry-pick ins_compl_len() -> get_compl_len() from patch 8.2.4001.

Revert a71c5e9eb9: ctrl_x_mode is no
longer a global variable, so l_ctrl_x_mode is no longer needed.

src/nvim/option.c

@@ -49,6 +49,7 @@
 #include "nvim/highlight.h"
 #include "nvim/highlight_group.h"
 #include "nvim/indent_c.h"
+#include "nvim/insexpand.h"
 #include "nvim/keycodes.h"
 #include "nvim/macros.h"
 #include "nvim/mapping.h"