Quiet 'visibility' warning for qthread_tos() and qthread_bos()

Quiet "‘visibility’ attribute ignored on non-class types" warnings for
qthread_tos() and qthread_bos().

I think this change should be made for all functions, but these are the only
ones that currently produce the warning. See #50 for more info

Author: ronawho

src/qthread.c

@@ -1881,15 +1881,15 @@ unsigned API_FUNC qthread_size_tasklocal(void)
     return f->rdata->tasklocal_size ? f->rdata->tasklocal_size : qlib->qthread_tasklocal_size;
 } /*}}}*/
 
-void* API_FUNC qthread_tos(void)
+API_FUNC void* qthread_tos(void)
 {
     const qthread_t *f = qthread_internal_self();
 
     return f->rdata->stack;
 }
 
 
-void* API_FUNC qthread_bos(void)
+API_FUNC void* qthread_bos(void)
 {
     const qthread_t *f = qthread_internal_self();