Fix missing import
Run into this while compiling on Debian 10:
kpa-thumbnailtool/TestThumbnailCacheConverter.cpp:46:17:
error: variable ‘QTextStream err’ has initializer but incomplete type
Run into this while compiling on Debian 10:
kpa-thumbnailtool/TestThumbnailCacheConverter.cpp:46:17:
error: variable ‘QTextStream err’ has initializer but incomplete type