tests: Include unistd.h only if available

Include the header in tests/torture_pki.c only if it is available.  It
is not available in Windows environment.

Signed-off-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

tests/torture_pki.c

@@ -4,7 +4,10 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
+
+#ifdef HAVE_UNISTD_H
 #include <unistd.h>
+#endif
 
 #include "torture_pki.h"