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/lua/stdlib.c

@@ -5,8 +5,8 @@
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
+#include <stdio.h>
 #include <string.h>
-#include <sys/types.h>
 
 #ifdef NVIM_VENDOR_BIT
 # include "bit.h"