cppzmq/czmq: Update comment for needs threads support

Comment 7e37d235 "zeromq: needs threads" adds BR2_TOOLCHAIN_HAS_THREADS
dependency but forgot to add corresponding comment for cppzmq and czmq. Fix it.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>