Fix two GCC 16 issues (#2164)

* [test] Mark unused variables

Found by GCC (-Wunused-but-set-variable).

* [utils] Add missing include for export macro

include/benchmark/utils.h

diff --git a/include/benchmark/utils.h b/include/benchmark/utils.h
index 2be0d3f..681d938 100644
--- a/include/benchmark/utils.h
+++ b/include/benchmark/utils.h
@@ -19,6 +19,7 @@
 #include <type_traits>
 #include <utility>
 
+#include "benchmark/export.h"
 #include "benchmark/macros.h"
 
 namespace benchmark {

test/basic_test.cc

diff --git a/test/basic_test.cc b/test/basic_test.cc
index e1db1cb..a20e5b4 100644
--- a/test/basic_test.cc
+++ b/test/basic_test.cc
@@ -1,4 +1,5 @@
 
+#include "benchmark/macros.h"
 #include "benchmark/registration.h"
 #include "benchmark/state.h"
 #include "benchmark/types.h"
@@ -150,7 +151,7 @@
 template <typename T>
 void BM_OneTemplateFunc(benchmark::State& state) {
   auto arg = state.range(0);
-  T sum = 0;
+  BENCHMARK_UNUSED T sum = 0;
   for (auto _ : state) {
     sum += static_cast<T>(arg);
   }
@@ -161,8 +162,8 @@
 template <typename A, typename B>
 void BM_TwoTemplateFunc(benchmark::State& state) {
   auto arg = state.range(0);
-  A sum = 0;
-  B prod = 1;
+  BENCHMARK_UNUSED A sum = 0;
+  BENCHMARK_UNUSED B prod = 1;
   for (auto _ : state) {
     sum += static_cast<A>(arg);
     prod *= static_cast<B>(arg);