only include sys/select.h if present, fix mingw compilation

Originally committed as revision 15420 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/rtsp.c

@@ -26,7 +26,9 @@
 #include "avformat.h"
 
 #include <sys/time.h>
+#ifdef HAVE_SYS_SELECT_H
 #include <sys/select.h>
+#endif
 #include <strings.h>
 #include "network.h"
 #include "rtsp.h"

libavformat/tcp.c

@@ -22,7 +22,9 @@
 #include <unistd.h>
 #include "network.h"
 #include "os_support.h"
+#ifdef HAVE_SYS_SELECT_H
 #include <sys/select.h>
+#endif
 #include <sys/time.h>
 
 typedef struct TCPContext {