rtspenc: include os_support.h for system without HAVE_POLL_H

fix compile on OS/2
Signed-off-by: Janne Grunau <janne-ffmpeg@jannau.net>
(cherry picked from commit a0788cc6)

libavformat/rtspenc.c

@@ -26,6 +26,7 @@
 #include <poll.h>
 #endif
 #include "network.h"
+#include "os_support.h"
 #include "rtsp.h"
 #include "internal.h"
 #include "libavutil/intreadwrite.h"