sm: Init nPth which might be used by some helper code.

* sm/gpgsm.c: Include npth.h.
(main): Init nPth.

Actually we currently do not use extra threads but we need to link to
the npth version of the common functions because there is an option to
use threads. Now, without initialized npth, we could experience a
crash in gnupg_sleep, which uses npth_usleep in the npth based common
functions.

• Fixes-commit: 046f419f806036248c058c4bd44368f8596287b7