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/eval/vars.c

@@ -7,6 +7,7 @@
 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/types.h>
 
 #include "nvim/ascii.h"
 #include "nvim/autocmd.h"