Fix std::mutex declaration

/run/build/BambuStudio/src/libslic3r/Preset.hpp:772:10: error: ‘mutex’ in namespace ‘std’ does not name a type
  772 |     std::mutex          m_mutex;
      |          ^~~~~
/run/build/BambuStudio/src/libslic3r/Preset.hpp:14:1: note: ‘std::mutex’ is defined in header ‘<mutex>’; did you forget to ‘#include <mutex>’?
   13 | #include "ProjectTask.hpp"
  +++ |+#include <mutex>
   14 |

src/libslic3r/Preset.hpp

@@ -5,6 +5,7 @@
 #include <set>
 #include <unordered_map>
 #include <functional>
+#include <mutex>
 #include <boost/filesystem/path.hpp>
 #include <boost/property_tree/ptree_fwd.hpp>