LibraryScanner: static_cast<int> qsizetype values for Qt6

[Be-ing]

[356/695] Building CXX object CMakeFiles/mixxx-lib.dir/src/library/scanner/libraryscanner.cpp.o
../src/library/scanner/libraryscanner.cpp: In member function ‘void LibraryScanner::slotFinishUnhashedScan()’:
../src/library/scanner/libraryscanner.cpp:427:14: warning: format ‘%d’ expects argument of type ‘int’, but argument 4 has type ‘qsizetype’ {aka ‘long long int’} [-Wformat=]
  427 |            "%d unchanged directories. "
      |             ~^
      |              |
      |              int
      |             %lld
......
  432 |            m_scannerGlobal->verifiedDirectories().size(),
      |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                                       |
      |                                                       qsizetype {aka long long int}
../src/library/scanner/libraryscanner.cpp:429:14: warning: format ‘%d’ expects argument of type ‘int’, but argument 6 has type ‘qsizetype’ {aka ‘long long int’} [-Wformat=]
  429 |            "%d tracks verified from changed/added directories. "
      |             ~^
      |              |
      |              int
      |             %lld
......
  434 |            m_scannerGlobal->verifiedTracks().size(),
      |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                                  |
      |                                                  qsizetype {aka long long int}
../src/library/scanner/libraryscanner.cpp:430:14: warning: format ‘%d’ expects argument of type ‘int’, but argument 7 has type ‘qsizetype’ {aka ‘long long int’} [-Wformat=]
  430 |            "%d new tracks.",
      |             ~^
      |              |
      |              int
      |             %lld
......
  435 |            m_scannerGlobal->addedTracks().size());
      |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                               |
      |                                               qsizetype {aka long long int}

[uklotzde]

LGTM

The usage of qDebug() with the format string is uncommon, but let's leave it as is.