Fix compilation when SCM_I_PTHREAD_ONCE_INIT is not aggregate

* libguile/scmsigs.c (scm_i_signals_post_fork): It's possible for
SCM_I_PTHREAD_ONCE_INIT to be a braced initializer.  In that case when
this initializer is re-used as an assignment, it needs the type to
instead be treated as a C11 literal.  Fixes compilation on Mac OS.

Thanks to Tony Garnock-Jones for the patch.

libguile/scmsigs.c

@@ -302,7 +302,7 @@ scm_i_signals_post_fork ()
     }
 
 #if SCM_USE_PTHREAD_THREADS
-  once = SCM_I_PTHREAD_ONCE_INIT;
+  once = (scm_i_pthread_once_t) SCM_I_PTHREAD_ONCE_INIT;
 #endif
   if (active)
     scm_i_ensure_signal_delivery_thread ();