Fix compile error with clang -Werror -Wunused-template (#2268)

This sysctl override is only used on some OS'es
so it causes compile error with latest LLVM.

src/sysinfo.cc

diff --git a/src/sysinfo.cc b/src/sysinfo.cc
index ca32daa..a291e10 100644
--- a/src/sysinfo.cc
+++ b/src/sysinfo.cc
@@ -149,7 +149,7 @@
   }
 
   template <class T, int N>
-  std::array<T, N> GetAsArray() {
+  BENCHMARK_MAYBE_UNUSED std::array<T, N> GetAsArray() {
     const int arr_size = sizeof(T) * N;
     BM_CHECK_LE(arr_size, size);
     std::array<T, N> arr;
@@ -210,7 +210,8 @@
 }
 
 template <class Tp, size_t N>
-bool GetSysctl(std::string const& name, std::array<Tp, N>* out) {
+BENCHMARK_MAYBE_UNUSED bool GetSysctl(std::string const& name,
+                                      std::array<Tp, N>* out) {
   auto buff = GetSysctlImp(name);
   if (!buff) return false;
   *out = buff.GetAsArray<Tp, N>();