Fix Clang++ 15.0 compiler issue with spiel_utils

[giogix2]

This PR fixes one of the issues reported in #1293.
The compiler fails when building the Dockerfile.base. Specifically, the issue occurs when the compiler Clang++ 15.0.0 is used (Note: no other compilers were tested, so it's unclear if the issue happens with other compilers or settings).

The error looks as the following

3.450 In file included from /repo/open_spiel/spiel_utils.cc:15:
3.452 /repo/open_spiel/../open_spiel/spiel_utils.h:51:59: error: no template named 'unique_ptr' in namespace 'std'
3.452 std::ostream& operator<<(std::ostream& stream, const std::unique_ptr<T>& v);
3.452                                                      ~~~~~^
3.463 /repo/open_spiel/../open_spiel/spiel_utils.h:88:59: error: no template named 'unique_ptr' in namespace 'std'
3.463 std::ostream& operator<<(std::ostream& stream, const std::unique_ptr<T>& v) {

and it can be fixed by explicitly importing the library memory in spiel_utils.h.

[lanctot]

Thanks!!

[lanctot]

Same here please: Can you pull changes from master and push the merge commit to trigger the CI tests again?