The compiler is telling you that you probably should do this:
cout << (x+x); to make your intentions more clear.
However, + has a higher precedent than <<, so it isn't actually necessary. Your compiler is just being overly cautious. Maybe you can try reducing the warning level in your compiler settings... or just put parenthesis around it.
I would do the latter -- I prefer putting parenthesis wherever there is possibility for confusion. Not everyone has the order of precedence memorized, so I like to clarify it whenever possible.