fix build on FreeBSD: eliminate implicitly declared functions

Change-Id: I4f7222f19e4d7129a5cef828a28dd12a40824a59
diff --git a/src/hlr.c b/src/hlr.c
index e87ebb6..86c5dcc 100644
--- a/src/hlr.c
+++ b/src/hlr.c
@@ -116,6 +116,8 @@
 	uint8_t *peer;
 };
 
+void lu_op_tx_insert_subscr_data(struct lu_operation *luop);
+
 void lu_op_statechg(struct lu_operation *luop, enum lu_state new_state)
 {
 	enum lu_state old_state = luop->state;