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/eval/decode.c

@@ -1,20 +1,31 @@
 // This is an open source non-commercial project. Dear PVS-Studio, please check
 // it. PVS-Studio Static Code Analyzer for C, C++ and C#: http://www.viva64.com
 
-#include <msgpack.h>
+#include <assert.h>
+#include <msgpack/object.h>
+#include <stdbool.h>
 #include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
 
 #include "klib/kvec.h"
 #include "nvim/ascii.h"
-#include "nvim/charset.h"  // vim_str2nr
+#include "nvim/charset.h"
 #include "nvim/eval.h"
 #include "nvim/eval/decode.h"
 #include "nvim/eval/encode.h"
 #include "nvim/eval/typval.h"
-#include "nvim/globals.h"
+#include "nvim/eval/typval_defs.h"
+#include "nvim/garray.h"
+#include "nvim/gettext.h"
+#include "nvim/hashtab.h"
 #include "nvim/macros.h"
+#include "nvim/mbyte.h"
+#include "nvim/memory.h"
 #include "nvim/message.h"
-#include "nvim/vim.h"  // OK, FAIL
+#include "nvim/types.h"
+#include "nvim/vim.h"
 
 /// Helper structure for container_struct
 typedef struct {