Bundle desktop-file-validate and mksquashfs

Closes #641

Author: probonopd

tests/tests-ci.sh

@@ -26,7 +26,7 @@ make -j$(nproc)
 set -e
 
 mkdir -p linuxdeployqt.AppDir/usr/{bin,lib}
-cp /usr/bin/{patchelf,desktop-file-validate} /usr/local/bin/{appimagetool,zsyncmake} linuxdeployqt.AppDir/usr/bin/
+cp /usr/bin/{patchelf,desktop-file-validate} /usr/local/bin/{appimagetool,desktop-file-validate,mksquashfs,zsyncmake} linuxdeployqt.AppDir/usr/bin/
 cp ./bin/linuxdeployqt linuxdeployqt.AppDir/usr/bin/
 # cp -r /usr/local/lib/appimagekit linuxdeployqt.AppDir/usr/lib/
 chmod +x linuxdeployqt.AppDir/AppRun