build: allow IWYU to fix includes for all .c files

Allow Include What You Use to remove unnecessary includes and only
include what is necessary. This helps with reducing compilation times
and makes it easier to visualise which dependencies are actually
required.

Work on https://github.com/neovim/neovim/issues/549, but doesn't close
it since this only works fully for .c files and not headers.

src/nvim/map.c

@@ -8,17 +8,16 @@
 //       khash.h does not make its own copy of the key or value.
 //
 
-#include <lauxlib.h>
-#include <lua.h>
 #include <stdbool.h>
 #include <stdlib.h>
 #include <string.h>
 
+#include "auto/config.h"
 #include "klib/khash.h"
+#include "nvim/gettext.h"
 #include "nvim/map.h"
 #include "nvim/map_defs.h"
 #include "nvim/memory.h"
-#include "nvim/vim.h"
 
 #define cstr_t_hash kh_str_hash_func
 #define cstr_t_eq kh_str_hash_equal