Remove duplicate includes

cache_varnishd.h already includes:
- sys/socket.h
- string.h

Spotted by local Flexelint install on Linux

bin/varnishd/cache/cache_acceptor.c

@@ -38,7 +38,6 @@
 #include <stdlib.h>
 #include <netinet/in.h>
 #include <netinet/tcp.h>
-#include <sys/socket.h>
 
 #include "cache_varnishd.h"
 

bin/varnishd/cache/cache_backend_probe.c

@@ -41,7 +41,6 @@
 
 #include <poll.h>
 #include <stdio.h>
-#include <string.h>
 #include <stdlib.h>
 
 #include "cache_varnishd.h"