avutil/bprint:ædd io.h, try to fix msvc build

Suggested-by: jamrial
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/bprint.c

@@ -30,6 +30,9 @@
 #include "error.h"
 #include "mem.h"
 
+#if HAVE_IO_H
+#include <io.h>
+#endif
 #if HAVE_UNISTD_H
 #include <unistd.h>
 #endif