protect unistd.h with #if HAVE_UNISTD_H in code from recent av_bprint_fd_contents() patches

Should fix build failure on MSVC

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

libavdevice/lavfi.c

@@ -27,7 +27,6 @@
 
 #include <float.h>              /* DBL_MIN, DBL_MAX */
 #include <fcntl.h>              /* O_RDONLY */
-#include <unistd.h>             /* close() */
 
 #include "libavutil/bprint.h"
 #include "libavutil/channel_layout.h"
@@ -43,6 +42,10 @@
 #include "libavformat/internal.h"
 #include "avdevice.h"
 
+#if HAVE_UNISTD_H
+#include <unistd.h>             /* close() */
+#endif
+
 typedef struct {
     AVClass *class;          ///< class for private options
     char          *graph_str;