Core/Analytics: Make MakeUniqueId() a const member function

This function doesn't modify instance state, so we can mark it as const.

Source/Core/Core/Analytics.cpp

@@ -101,7 +101,7 @@ void DolphinAnalytics::GenerateNewIdentity()
   SConfig::GetInstance().SaveSettings();
 }
 
-std::string DolphinAnalytics::MakeUniqueId(std::string_view data)
+std::string DolphinAnalytics::MakeUniqueId(std::string_view data) const
 {
   std::array<u8, 20> digest;
   const auto input = std::string{m_unique_id}.append(data);