Fix: std::string fName = ...c_str(); -> ...string();

This fixes:
mastercore.cpp: In function ‘int load_most_relevant_state()’:
mastercore.cpp:1827:56: error: conversion from ‘const value_type* {aka const wchar_t*}’ to non-scalar type ‘std::string {aka std::basic_string<char>}’ requested
Merge pull request #134 from dexX7/mcore-fix-c_str-string

Fix: std::string fName = ...c_str(); -> ...string();