cmake: link Threads::Threads instead of CMAKE_THREAD_LIBS_INIT

cmake: link Threads::Threads instead of CMAKE_THREAD_LIBS_INIT

CMAKE_THREAD_LIBS_INIT can be empty. On newer platforms, this leads to
an error:

  CMake Error at cmake/modules/Buildpmem.cmake:47 (set_target_properties):
  set_target_properties called with incorrect number of arguments.

This is because glibc 2.34 merged libpthread.so into libc.so.
Fedora 35 and RHEL 9 have glibc 2.34.

Fixes: https://tracker.ceph.com/issues/52353
Signed-off-by: Ken Dreyer <kdreyer@redhat.com>
Co-authored-by: Kefu Chai <kchai@redhat.com>