add POSIX semaphore example

Author: Vladimir Kotal

threads.tex

@@ -1400,6 +1400,7 @@
 unnamed.
 \item The semaphore functions adhere to the classical UNIX semantics --
 they return -1 on error and set \texttt{errno}.
+\item Example: \example{semaphores/sem.c}
 \end{itemize}
 
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