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/api/buffer.c

@@ -3,25 +3,30 @@
 
 // Some of this code was adapted from 'if_py_both.h' from the original
 // vim source
-#include <lauxlib.h>
-#include <limits.h>
-#include <stdbool.h>
-#include <stdint.h>
-#include <stdlib.h>
 
+#include <assert.h>
+#include <lauxlib.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <string.h>
+
+#include "klib/kvec.h"
+#include "lua.h"
 #include "nvim/api/buffer.h"
 #include "nvim/api/private/defs.h"
 #include "nvim/api/private/helpers.h"
+#include "nvim/ascii.h"
 #include "nvim/autocmd.h"
 #include "nvim/buffer.h"
+#include "nvim/buffer_defs.h"
 #include "nvim/buffer_updates.h"
 #include "nvim/change.h"
 #include "nvim/cursor.h"
-#include "nvim/decoration.h"
 #include "nvim/drawscreen.h"
 #include "nvim/ex_cmds.h"
-#include "nvim/ex_docmd.h"
 #include "nvim/extmark.h"
+#include "nvim/globals.h"
 #include "nvim/lua/executor.h"
 #include "nvim/mapping.h"
 #include "nvim/mark.h"
@@ -29,9 +34,10 @@
 #include "nvim/memory.h"
 #include "nvim/move.h"
 #include "nvim/ops.h"
+#include "nvim/pos.h"
+#include "nvim/types.h"
 #include "nvim/undo.h"
 #include "nvim/vim.h"
-#include "nvim/window.h"
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "api/buffer.c.generated.h"