libbb: include <crypt.h> only if necessary

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/libbb/pw_encrypt.c b/libbb/pw_encrypt.c
index 86455cd..47c2069 100644
--- a/libbb/pw_encrypt.c
+++ b/libbb/pw_encrypt.c
@@ -6,7 +6,9 @@
  *
  * Licensed under GPLv2 or later, see file LICENSE in this source tree.
  */
+#if !ENABLE_USE_BB_CRYPT
 #include <crypt.h>
+#endif
 #include "libbb.h"
 
 /* static const uint8_t ascii64[] ALIGN1 =