log: Only include unistd.h if configure found it

MSVC has isatty (in io.h), but not unistd.h. (isatty isn't called
at all for windows, since there's a special case block for that.)
Signed-off-by: Martin Storsjö <martin@martin.st>

libavutil/log.c

@@ -26,7 +26,7 @@
 
 #include "config.h"
 
-#if HAVE_ISATTY
+#if HAVE_UNISTD_H
 #include <unistd.h>
 #endif
 #include <stdlib.h>