ffmpeg: Only include unistd.h if it exists

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -31,8 +31,13 @@
 #include <errno.h>
 #include <limits.h>
 #if HAVE_ISATTY
+#if HAVE_IO_H
+#include <io.h>
+#endif
+#if HAVE_UNISTD_H
 #include <unistd.h>
 #endif
+#endif
 #include "libavformat/avformat.h"
 #include "libavdevice/avdevice.h"
 #include "libswscale/swscale.h"