appimage: use mlt v6 branch

packaging/appimage/3rdparty/ext_mlt/CMakeLists.txt

@@ -2,6 +2,7 @@ ExternalProject_Add(
   ext_mlt
   DOWNLOAD_DIR ${EXT_DOWNLOAD_DIR}
   GIT_REPOSITORY https://github.com/mltframework/mlt.git
+  GIT_TAG v6
 
   CONFIGURE_COMMAND <SOURCE_DIR>/configure --prefix=${EXT_INSTALL_DIR} ${AT_HOST} ${AT_SHARED_ONLY} --enable-gpl --enable-gpl3 --enable-opencv --enable-opengl --enable-lumas --avformat-swscale
   BUILD_IN_SOURCE 1