[Bug c++/78656] Fix-it suggestion for std::alocator doesn't include std::allocator

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78656

--- Comment #2 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
Candidate patch:
  https://gcc.gnu.org/ml/gcc-patches/2017-01/msg00143.html