To understand why your code does not compile, reflect how you have declared Foo class and how move semantics is generally implemented.
Declaring a const unique_ptr<T> p, you mean that p itself will be never modified, but you could still modify the pointed-to object because of T is not const.
But move works on an opposite assumption. This feature uses the idea that is allowed stealing resources from objects and leave them in a empty state (if an empty state make sense). If can be useful, think move as a sort of 'destructive' copy for the moved object.
Writing std::move(foo.p), basically you steal the resource pointed by foo.p and leave it in a safe state, that means assign foo.p to NULL. But foo.p was declared as const, so the operation is not permitted.
Please consider that in your case you don't need to declare p as a const unique_ptr<int>. Simply declare it as unique_ptr<int> and make sure that member functions are declared as const and non-member functions take it asconst unique_ptr<int> p& parameter. In this way you are sure that p will never change along the object lifetime (except in case of move operation).