CMake: Fix build on gles Debian CI due to optional deps missing

We need to do some target checks before adding the subdirectories,
because some of the optional dependencies might be missing.

Avoids errors like the following in the Debian CI packaging:

  CMake Error at /opt/qt-6.13.0/aarch64-linux-gnu/lib/cmake/Qt6/
    QtFindPackageHelpers.cmake:715 (get_target_property):
   get_target_property() called with non-existent target
   "Qt6::QuickTimeline".
 Call Stack (most recent call first):
   QtTargetHelpers.cmake:300
    (qt_internal_register_target_dependencies)
   QtModuleHelpers.cmake:716 (qt_internal_extend_target)
   src/lottiegenerator/CMakeLists.txt:4 (qt_internal_add_module)

Pick-to: 6.12
Change-Id: Icfb9f2181e85caac059738bb45acf7226dced712
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Simo Fält <simo.falt@qt.io>