db: The forward declaration of the enum does not work with C++

We need to include this directly to help LCR in building.
diff --git a/openbsc/include/openbsc/db.h b/openbsc/include/openbsc/db.h
index a939b0d..fd9181e 100644
--- a/openbsc/include/openbsc/db.h
+++ b/openbsc/include/openbsc/db.h
@@ -21,6 +21,7 @@
 #define _DB_H
 
 #include <sys/types.h>
+#include "gsm_subscriber.h"
 
 struct gsm_equipment;
 struct gsm_network;
@@ -29,8 +30,6 @@
 struct gsm_sms;
 struct gsm_subscriber;
 
-enum gsm_subscriber_field;
-
 /* one time initialisation */
 int db_init(const char *name);
 int db_prepare();