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/vim.h

@@ -1,7 +1,7 @@
 #ifndef NVIM_VIM_H
 #define NVIM_VIM_H
 
-#include "nvim/pos.h"  // for linenr_T, MAXCOL, etc...
+#include "nvim/pos.h"
 #include "nvim/types.h"
 
 // Some defines from the old feature.h