Rename basic-forward.h, sd-forward.h, and shared-forward.h to forward.h

Follow-up for 74d392ed1b.

src/sysupdate/sysupdate-config.h

@@ -2,7 +2,7 @@
 #pragma once
 
 #include "conf-parser-forward.h"
-#include "shared-forward.h"
+#include "forward.h"
 
 CONFIG_PARSER_PROTOTYPE(config_parse_url_specifiers);
 CONFIG_PARSER_PROTOTYPE(config_parse_url_specifiers_many);

src/sysupdate/sysupdate-forward.h

@@ -3,7 +3,7 @@
 
 /* IWYU pragma: always_keep */
 
-#include "shared-forward.h" /* IWYU pragma: export */
+#include "forward.h" /* IWYU pragma: export */
 
 typedef struct Context Context;
 typedef struct Instance Instance;

src/sysupdate/sysupdate-update-set-flags.h

@@ -1,7 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include "shared-forward.h"
+#include "forward.h"
 
 typedef enum UpdateSetFlags {
         UPDATE_NEWEST     = 1 << 0,