build: rework IWYU mapping files

Create mapping to most of the C spec and some POSIX specific functions.
This is more robust than relying files shipped with IWYU.

src/nvim/msgpack_rpc/channel.c

@@ -7,7 +7,6 @@
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
-#include <uv.h>
 
 #include "klib/kvec.h"
 #include "nvim/api/private/defs.h"