Update INIReader.cpp

[xkzl]

Update INIReader::INIReader prototype implementation to match header implementation.
This update is removing the warning when generating Doxygen documentation.

[benhoyt]

Thanks for the contribution, but I don't think I'll merge this. If we were going to change it here, we should change it everywhere and remove the using std::string line (but I'd rather not do that -- the using declaration is intentional as it keeps the code shorter). If Doxygen can't figure out that std::string and string (with using std::string) are the same thing, that's a Doxygen problem.