log: include unistd.h only when needed

The only symbol this file uses from unistd.h is isatty().  By
including the header only when this function is used, the file
can be built on systems without unistd.h (which presumably also
lack isatty).
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavutil/log.c

@@ -24,7 +24,11 @@
  * logging functions
  */
 
+#include "config.h"
+
+#if HAVE_ISATTY
 #include <unistd.h>
+#endif
 #include <stdlib.h>
 #include "avstring.h"
 #include "avutil.h"