add include

Author: vladak

intro.tex

@@ -1864,7 +1864,7 @@
     \item returning from \texttt{main()} or calling \texttt{exit()} terminates the
     process
     \item standard return values are \verb#EXIT_SUCCESS# (0) and
-    \verb#EXIT_FAILURE# (1)
+    \verb#EXIT_FAILURE# (1) (\texttt{stdlib.h})
     \end{itemize}
 \end{itemize}