pthread_once: fix mutex_unlock to allow recursion

Unlock the __once_mutex before calling the specified function
to avoid deadlock on recursive calls, as apparently can
happen in the C++ STL.

Fix from Robert David <davidrob@fit.cvut.cz>.

pthread.c

diff --git a/pthread.c b/pthread.c
index 8310595..bfca88c 100644
--- a/pthread.c
+++ b/pthread.c
@@ -716,8 +716,8 @@
     return 0;
   }
   *once = PTHREAD_ONCE_INIT+1;
-  init();
   pthread_mutex_unlock(&__once_mutex);
+  init();
   return 0;
 }