Use wrapped __assuan_usleep for _assuan_npth_usleep.

* src/assuan.h.in (_assuan_npth_usleep): Wrap __assuan_usleep.

Assuan should support system hooks for nPth before npth_init.

Before the call of npth_init, npth_unprotect and npth_protect do
nothing, whiile npth_usleep may crash.

No use of npth_usleep is better.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>