vlc_messages: expose struct vlc_logger definition

The structure is already used in the Rust bindings and has as low
dependencies as possible to anything. Exposing it allows creating logger
that will work in a minimal test environment.

include/vlc_messages.h

@@ -121,7 +121,9 @@ VLC_API const char *vlc_strerror_c(int);
  * @{
  */
 
-struct vlc_logger;
+struct vlc_logger {
+    const struct vlc_logger_operations *ops;
+};
 
 VLC_API void vlc_Log(struct vlc_logger *const *logger, int prio,
                      const char *type, const char *module,

src/input/test/es_out.c

@@ -329,10 +329,6 @@ static const struct vlc_logger_operations test_logger_operations = {
     .log = LogText,
 };
 
-struct vlc_logger {
-    const struct vlc_logger_operations *ops;
-};
-
 static void test_playback(void)
 {
     vlc_list_init(&opened_decoders);

src/misc/messages.c

@@ -82,10 +82,6 @@ static void vlc_LogSpam(vlc_object_t *obj)
     msg_Dbg(obj, "configured with %s", CONFIGURE_LINE);
 }
 
-struct vlc_logger {
-    const struct vlc_logger_operations *ops;
-};
-
 static void vlc_vaLogCallback(vlc_logger_t *logger, int type,
                               const vlc_log_t *item, const char *format,
                               va_list ap)