Use const refs for strings Sniffer::setFilter

daemon/include/Sniffer.h

@@ -46,7 +46,7 @@ public:
    *
    * @param filterString pcap filter string
    */
-  void setFilter(std::string filterString);
+  void setFilter(const std::string &filterString);
 
 private:
   /**

daemon/src/Sniffer.cpp

@@ -21,7 +21,7 @@ void Sniffer::startSniffing() {
   sniffer->sniff_loop(make_sniffer_handler(this, &Sniffer::handle));
 }
 
-void Sniffer::setFilter(std::string filterString) {
+void Sniffer::setFilter(const std::string &filterString) {
   sniffer->set_filter(filterString);
 }