[gdb/build] Fix build with gcc 9

[gdb/build] Fix build with gcc 9

Since commit a691853148f ("gdb/python: introduce gdbpy_registry"), when
building gdb with gcc 9, I run into:
...
In file included from gdb/varobj.c:38:0:
gdb/python/python-internal.h:1211:47: error: expected ‘;’ before ‘<’ token
   using StorageKey = typename registry<O>::key<Storage>;
                                               ^
...
due to this code:
...
template <typename Storage>
class gdbpy_registry
{
  ...

  template<typename O>
  using StorageKey = typename registry<O>::key<Storage>;

  template<typename O>
  Storage *get_storage (O *owner, const StorageKey<O> &key) const
  { ... }

  ...
}
...

As an experiment, I tried out eliminating the type alias:
...
  template<typename O>
  Storage *get_storage (O *owner,
                        const typename registry<O>::key<Storage> &key) const
  { ... }
...
and got instead:
...
In file included from gdb/varobj.c:38:0:
gdb/python/python-internal.h:1211:63: error: non-template ‘key’ used as template
  Storage *get_storage (O *owner,
                        const typename registry<O>::key<Storage> &key) const
                                                     ^~~
gdb/python/python-internal.h:1211:63: note: use ‘registry<O>::template key’ \
  to indicate that it is a template
...

Following that suggestion, I tried:
...
  template<typename O>
  Storage *
  get_storage (O *owner,
               const typename registry<O>::template key<Storage> &key) const
  { ... }
...
which fixed the problem.

Likewise, adding the template keyword in the type alias fixes the original
problem, so fix it like that.

Tested on x86_64-linux.