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/change.h

@@ -1,8 +1,8 @@
 #ifndef NVIM_CHANGE_H
 #define NVIM_CHANGE_H
 
-#include "nvim/buffer_defs.h"  // for buf_T
-#include "nvim/pos.h"  // for linenr_T
+#include "nvim/buffer_defs.h"
+#include "nvim/pos.h"
 
 // flags for open_line()
 #define OPENLINE_DELSPACES  0x01  // delete spaces after cursor