Fix build when scm_t_timespec is different from struct timespec

Fix build when scm_t_timespec is different from struct timespec

Reported by Roland Haeder.  The declaration and definition of
scm_pthread_cond_timedwait were using possibly different types for the
third arg.

* THANKS: Added Roland Haeder.

* libguile/threads.h (scm_pthread_cond_timedwait): Use scm_t_timespec
  for third arg rather than struct timespec, for consistency with the
  function implementation.