chore: rule of 3 for strdup guard (#3905)

Author: Skylion007

include/pybind11/pybind11.h

@@ -312,6 +312,10 @@ class cpp_function : public function {
     // along the way.
     class strdup_guard {
     public:
+        strdup_guard() = default;
+        strdup_guard(const strdup_guard &) = delete;
+        strdup_guard &operator=(const strdup_guard &) = delete;
+
         ~strdup_guard() {
             for (auto *s : strings) {
                 std::free(s);