CMake: Link xz against Threads::Threads if using pthreads

CMake: Link xz against Threads::Threads if using pthreads

The liblzma target was recently changed to link against Threads::Threads
with the PRIVATE keyword. I had forgotten that xz itself depends on
pthreads too due to pthread_sigmask(). Thus, the build broke when
building shared liblzma and pthread_sigmask() wasn't in libc.

Thanks to Peter Seiderer for the bug report.

Fixes: ac05f1b0d7cda1e7ae79775a8dfecc54601d7f1c
Fixes: https://github.com/tukaani-project/xz/issues/129#issuecomment-2204522994