ck_ht: key_length/key are only needed if PP is enabled.

src/ck_ht.c

@@ -476,8 +476,10 @@ restart:
 			continue;
 
 		if (table->mode == CK_HT_MODE_BYTESTRING) {
+#ifdef CK_HT_PP
 			void *key;
 			uint16_t key_length;
+#endif
 
 			key = ck_ht_entry_key(previous);
 			key_length = ck_ht_entry_key_length(previous);