libopenmpt: fix build on gcc-12

`gcc-12` cleaned up implicit header includes and now requires
explicit '<memory>' for 'std::unique_ptr<>':

    openmpt123/openmpt123.cpp:194:14: error: 'unique_ptr' in namespace 'std' does not name a template type
      194 |         std::unique_ptr<file_audio_stream_base> impl;
          |              ^~~~~~~~~~
    openmpt123/openmpt123.cpp:109:1: note: 'std::unique_ptr' is defined in header '<memory>'; did you forget to '#include <memory>'?
      108 | #include "openmpt123_waveout.hpp"
      +++ |+#include <memory>
      109 |

0 files changed, 0 insertions, 0 deletions