Define MAXNS locally for netd_benchmark

The MAXNS was originally defined in libnetd_resolv_headers, which is
moving to private for minimizing exposed symbols on DNS Resolver.

Bug: 144407448
Test: atest under system/netd
Change-Id: If6d1c485d66d114216812d9f8b31090317f383c7
diff --git a/tests/benchmarks/dns_benchmark.cpp b/tests/benchmarks/dns_benchmark.cpp
index d25f536..6843bfe 100644
--- a/tests/benchmarks/dns_benchmark.cpp
+++ b/tests/benchmarks/dns_benchmark.cpp
@@ -44,12 +44,12 @@
 
 #include "NetdClient.h"
 #include "dns_responder_client.h"
-#include "netd_resolv/params.h"  // MAXNS
 
 using android::base::StringPrintf;
 
 constexpr int MIN_THREADS = 1;
 constexpr int MAX_THREADS = 32;
+constexpr int MAXNS = 4;
 
 class DnsFixture : public ::benchmark::Fixture {
 protected: