video_core: add missing header after 468bd9c1

src/video_core/shader_notify.cpp: In member function 'void VideoCore::ShaderNotify::MarkShaderComplete()':
src/video_core/shader_notify.cpp:33:10: error: 'unique_lock' is not a member of 'std'
   33 |     std::unique_lock lock{mutex};
      |          ^~~~~~~~~~~
src/video_core/shader_notify.cpp:6:1: note: 'std::unique_lock' is defined in header '<mutex>'; did you forget to '#include <mutex>'?
    5 | #include "video_core/shader_notify.h"
  +++ |+#include <mutex>
    6 |
src/video_core/shader_notify.cpp: In member function 'void VideoCore::ShaderNotify::MarkSharderBuilding()':
src/video_core/shader_notify.cpp:38:10: error: 'unique_lock' is not a member of 'std'
   38 |     std::unique_lock lock{mutex};
      |          ^~~~~~~~~~~
src/video_core/shader_notify.cpp:38:10: note: 'std::unique_lock' is defined in header '<mutex>'; did you forget to '#include <mutex>'?