Will get `cannot declare pointer to` error When include httplib.h

[Asa-gc]

when code like:

#include <iostream>
#include <string>
#include "httplib.h"

int func(int &_res){
    return 0;
}

int main(int argc, char *argv[])
{
    return 0;
}

will get this error:

error: cannot declare pointer to ‘int&’
[build]     5 | int func(int &_res){
[build]       |               ^~~~

The inspection found that it was because these code in <resolv.h>:159

__BEGIN_DECLS
extern struct __res_state *__res_state(void) __attribute__ ((__const__));
__END_DECLS
#define _res (*__res_state())

[yhirose]

Fix by #2280