htable: Fix declarations of variable ht_expired_cell

- Removed unused extern delaration in htable.c
- Reported by Ovidiu Sas

modules/htable/ht_var.c

@@ -26,7 +26,7 @@
 
 /* pkg copy */
 ht_cell_t *_htc_local=NULL;
-ht_cell_t *ht_expired_cell;
+extern ht_cell_t *ht_expired_cell;
 
 int pv_get_ht_cell(struct sip_msg *msg,  pv_param_t *param,
 		pv_value_t *res)

modules/htable/htable.c

@@ -52,7 +52,6 @@ MODULE_VERSION
 int  ht_timer_interval = 20;
 int  ht_db_expires_flag = 0;
 int  ht_enable_dmq = 0;
-extern ht_cell_t *ht_expired_cell;
 
 static int htable_init_rpc(void);