Make prefix filter operator const

Author: peterschrammel

src/util/prefix_filter.cpp

@@ -23,7 +23,7 @@ prefix_filtert::prefix_filtert(
 {
 }
 
-bool prefix_filtert::operator()(const std::string &value)
+bool prefix_filtert::operator()(const std::string &value) const
 {
   if(!included_prefixes.empty())
   {

src/util/prefix_filter.h

@@ -27,7 +27,7 @@ class prefix_filtert
   /// but doesn't match a prefix in `excluded_prefixes`.
   /// An empty vector of `included_prefixes` is treated as 'match all'.
   /// An empty vector of `excluded_prefixes` is treated as 'match nothing'.
-  bool operator()(const std::string &value);
+  bool operator()(const std::string &value) const;
 
 protected:
   std::vector<std::string> included_prefixes;