config: forward declare NMConfig in nm-types.h

src/nm-config.h

@@ -36,9 +36,9 @@ G_BEGIN_DECLS
 #define NM_IS_CONFIG_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass),  NM_TYPE_CONFIG))
 #define NM_CONFIG_GET_CLASS(obj)  (G_TYPE_INSTANCE_GET_CLASS ((obj),  NM_TYPE_CONFIG, NMConfigClass))
 
-typedef struct {
+struct _NMConfig {
 	GObject parent;
-} NMConfig;
+};
 
 typedef struct {
 	GObjectClass parent;

src/nm-types.h

@@ -26,6 +26,7 @@ typedef struct _NMActiveConnection   NMActiveConnection;
 typedef struct _NMVpnConnection      NMVpnConnection;
 typedef struct _NMActRequest         NMActRequest;
 typedef struct _NMAuthSubject        NMAuthSubject;
+typedef struct _NMConfig             NMConfig;
 typedef struct _NMConnectionProvider NMConnectionProvider;
 typedef struct _NMConnectivity       NMConnectivity;
 typedef struct _NMDBusManager        NMDBusManager;