[SYCL][PI] Restore find_package(Threads) to fix build issue in some env

[smaslov-intel]

Trying to fix regression after #6023
https://github.com/intel/llvm/runs/6164928984?check_suite_focus=true

CMake Error at D:/github/_work/llvm/llvm/src/sycl/cmake/modules/AddSYCL.cmake:8 (add_library):
Target "pi_level_zero" links to target "Threads::Threads" but the target
was not found. Perhaps a find_package() call is missing for an IMPORTED
target, or an ALIAS target is missing?
Call Stack (most recent call first):
D:/github/_work/llvm/llvm/src/sycl/cmake/modules/AddSYCL.cmake:41 (add_sycl_library)
D:/github/_work/llvm/llvm/src/sycl/plugins/level_zero/CMakeLists.txt:106 (add_sycl_plugin)

[pvchupin]

Abandoned in favor of #6060

[pvchupin]

It looks like #6060 restored post-commit but build fails in some configurations (e.g. in some older linux versions).