logging: Move GetLogCategory into Logger class

src/logging.cpp

@@ -224,7 +224,7 @@ static const std::unordered_map<BCLog::LogFlags, std::string> LOG_CATEGORIES_BY_
     }(LOG_CATEGORIES_BY_STR)
 };
 
-std::optional<BCLog::LogFlags> GetLogCategory(std::string_view str)
+std::optional<BCLog::LogFlags> BCLog::Logger::GetLogCategory(std::string_view str)
 {
     if (str.empty() || str == "1" || str == "all") {
         return BCLog::ALL;