fix warning

| ../git/src/netlink/nl_l3.cc:46:21: warning: loop variable 'v' creates a copy from type 'const basebox::nh_stub' [-Wrange-loop-construct]
|    46 |     for (const auto v : arg) {
|       |                     ^
| ../git/src/netlink/nl_l3.cc:46:21: note: use reference type to prevent copying
|    46 |     for (const auto v : arg) {
|       |                     ^
|       |                     &

Signed-off-by: Jonas Gorski <jonas.gorski@bisdn.de>

src/netlink/nl_l3.cc

@@ -43,7 +43,7 @@ template <class T, class A> struct hash<std::set<T, A>> {
   using result_type = std::size_t;
   result_type operator()(argument_type const &arg) const noexcept {
     result_type seed = 0;
-    for (const auto v : arg) {
+    for (const auto &v : arg) {
       hash_combine(seed, v);
     }
     return seed;