Add #include "os_support.h" to restore OS/2 support.

patch by Dave Yeo, daveryeo telus net

Originally committed as revision 11096 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffserver.c

@@ -28,6 +28,7 @@
 #include "avformat.h"
 #include "rtsp.h"
 #include "rtp.h"
+#include "os_support.h"
 
 #include <stdarg.h>
 #include <unistd.h>

libavformat/rtpproto.c

@@ -24,6 +24,7 @@
 #include <unistd.h>
 #include <stdarg.h>
 #include "network.h"
+#include "os_support.h"
 #include <fcntl.h>
 
 #define RTP_TX_BUF_SIZE  (64 * 1024)