hash table BUGFIX missing ly_ht declaration

Refs #2032
diff --git a/src/hash_table.h b/src/hash_table.h
index c45a6da..3733de2 100644
--- a/src/hash_table.h
+++ b/src/hash_table.h
@@ -28,6 +28,12 @@
 #include "log.h"
 
 /**
+ * @struct ly_ht
+ * @brief libyang hash table.
+ */
+struct ly_ht;
+
+/**
  * @brief Compute hash from (several) string(s).
  *
  * Usage: