Adjust function definition in vfs_bio.c to avoid clang 15 warnings

With clang 15, the following -Werror warning is produced:

    sys/kern/vfs_bio.c:3430:11: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
    buf_daemon()
              ^
               void

This is because buf_daemon() is declared with a (void) argument list,
but defined with an empty argument list. Make the definition match the
declaration.

MFC after:	3 days

sys/kern/vfs_bio.c

@@ -3427,7 +3427,7 @@ buf_daemon_shutdown(void *arg __unused, int howto __unused)
 }
 
 static void
-buf_daemon()
+buf_daemon(void)
 {
 	struct bufdomain *bd;
 	int speedupreq;