Common/FileUtil: Use std::string::data within ReadFileToString

With C++17, .data() for std::string now has a non-const overload, so we
can make use of that instead of taking the address of the first element.

Source/Core/Common/FileUtil.cpp

@@ -900,7 +900,7 @@ bool ReadFileToString(const std::string& filename, std::string& str)
     return false;
 
   str.resize(file.GetSize());
-  return file.ReadArray(&str[0], str.size());
+  return file.ReadArray(str.data(), str.size());
 }
 
 }  // namespace File