libitm: small update for C++20

C++20 DR 2237 disallows simple-template-id in cdtors, so you
can't write

    template<typename T>
    struct S {
      S<T>(); // should be S();
    };

This hasn't been a problem until now but I'm adding a warning about it
to -Wc++20-compat which libitm apparently uses.

libitm/ChangeLog:

	* containers.h (vector): Remove the template-id in constructors.