work-around res_search on older glibcs

`res_search` became a proper symbol only in glibc 2.34. Until that it
was re-defined in headers, hitting the (in-)famous
https://github.com/ziglang/zig/issues/9485

glibc 2.34+:

    resolv/resolv.h:int             res_search (const char *, int, int, unsigned char *, int)

Old glibc:

    resolv/resolv.h:#define res_search              __res_search

Also, remove the toplevel includes, as they should never be included in
the first place: the headers are included explicitly when needed.

test/cgo/cgo.go

@@ -4,8 +4,9 @@ package main
 // #include <unistd.h>
 // #include <fcntl.h>
 // #include <stdio.h>
+// #include <resolv.h>
 // char* hello() { return "hello, world"; }
-// void phello() { printf("%s, your lucky number is %p\n", hello(), fcntl); }
+// void phello() { printf("%s, your lucky numbers are %p and %p\n", hello(), fcntl, res_search); }
 import "C"
 
 func main() {