Fixed build error

httplib.h

@@ -356,7 +356,7 @@ struct case_ignore_equal {
 };
 
 struct case_ignore_hash {
-  constexpr size_t operator()(std::string_view key) const {
+  constexpr size_t operator()(const std::string &key) const {
     return hash_core(key.data(), key.size(), 0);
   }