scripts: valgrind: suppress missing clean-up in nct_new_thread()

Suppress memory leaks detected by valgrind originating from
nct_new_thread(), part of the native_sim CPU thread emulation.

This covers calls to posix_new_thread() as well, since this function simply
calls nct_new_thread (but may be optimized out by the compiler).

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>