Add missing cstring include in net/dns/address_info_test_util.cc

The code uses memset and memcpy provided by string.h. It does not
include this file however. On Linux this fails to build starting with
glibc 2.30.

Bug: 1074286
Change-Id: I549b02aad39ba0f022a2c84bfc2bb36c2ff31f6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2164508Reviewed-by: Eric Orth <ericorth@chromium.org>
Commit-Queue: Piotr Tworek <ptworek@vewd.com>
Cr-Commit-Position: refs/heads/master@{#762748}

net/dns/address_info_test_util.cc

@@ -4,6 +4,8 @@
 
 #include "net/dns/address_info_test_util.h"
 
+#include <cstring>
+
 #include "base/logging.h"
 #include "base/sys_byteorder.h"
 #include "net/base/sys_addrinfo.h"