Fix missing "crypt.h" compilation error on the FreeBSD

FreeBSD does not use crypt.h, but unistd.h which is already included

Signed-off-by: Alex Samorukov <samm@os2.kiev.ua>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/libbb/pw_encrypt.c b/libbb/pw_encrypt.c
index a60c33c..3463fd9 100644
--- a/libbb/pw_encrypt.c
+++ b/libbb/pw_encrypt.c
@@ -7,7 +7,9 @@
  * Licensed under GPLv2 or later, see file LICENSE in this source tree.
  */
 #if !ENABLE_USE_BB_CRYPT
-#include <crypt.h>
+# if !defined(__FreeBSD__)
+#  include <crypt.h>
+# endif
 #endif
 #include "libbb.h"