hlr_ussd.h: drop meaningless forward declaration

Change-Id: I70a5c7c83c2356b779fb1ea7ffe07ccc1e279c22
diff --git a/src/hlr_ussd.h b/src/hlr_ussd.h
index a190dc6..0414924 100644
--- a/src/hlr_ussd.h
+++ b/src/hlr_ussd.h
@@ -4,9 +4,8 @@
 
 #include <osmocom/core/linuxlist.h>
 #include <osmocom/gsm/gsup.h>
-#include "gsup_server.h"
 
-struct osmo_gsup_conn;
+#include "gsup_server.h"
 
 struct hlr_ussd_route {
 	/* g_hlr.routes */